What swimming suit options are acceptable for a self-conscious recreational swimmer in Belgium and Western Europe?

Websearch: ‘men’s Jammer’

I’d get these myself, but around here loose shorts are acceptable ¯\_(ツ)_/¯

Many of these swimming pools have shops or vending machines selling the appropriate attire.
You will have a lot more success in life if you learn to do these things, rather than leaving yourself with the phobia for all of your life.
Good search terms are "sungas" & "aquashorts" then click on "images" and choose the one you like.

They are in all the range of styles

• brief (5 to 10cm), classic (12cm), traditional (15cm)
• the square cuts of: Sungão (17cm), surf-short (18cm), boxer (19cm)
• longer versions: aquashort (20-30cm), jammer (40cm)

Learning to wear proper swimwear is something you go through,
not shorts that weigh 700 grams when wet.
Swimwear was only required from the 1870’s onwards in many countries.

The size and fit is an issue. Swim training in the pool requires
a firmer fit than relaxing at the beach. Optimal fit will vary depending on if it is for relaxation or an intensive workout. Also chlorine will slacken them over time so for chlorine avoid a too slacker a fit.

I am a between size person, and that has not been as easy.

The good thing about places like France, is that you can wear
just about what ever style of form fitting you like.
I am quite sure that the people there will be quite helpful
to you – the good thing is, it will be easier to learn.

Australia is also pretty cool about things, only behind the
continent, I think, but I also find myself tailoring the
style to the social setting, so as not to be burdened by
something more like aquashorts, where the social setting of pools
and beaches here – only needs briefs.

Spend some time thinking before you buy.
It may be good to have more than one style of sunga for different social situations, or depending on how you are feeling at the time.

SOME STEPS TO MAKING THE MOVE….
I have now had a proper read of your question’s backstory, and my experience with similar things – is that I have had to make a concerted effort, and gradually introduce myself…. You will need to overcome the chicken and egg problem of getting a good fit before you are seen by people, and gradually introduce yourself to being seen by others, when you have a style you are comfortable with.

PURCHASING..
The first step is to own a pair or pairs that fit you well. You can usually buy some good looking square-cuts, without swimming in the pool, where you can take them home and try them on. Online there are many also.
swimwear365 and ProSwimWear UK are examples of that.

FIT & STYLE..
A medium side height of about 15 to 16 centimeters makes a good style which is not so brief, but passes well with locals. Avoiding having a vertical seam that goes up the middle of the front will be more comfortable. Another thing to experiment with is colours and livery. Trunk style sungas are good, because if the fit is on the firm side, they will tolerate that and slacken with age.

BEING SEEN..
When you move to being seen in them, then this shall be a stage that may come relatively easily, or may take some doing. A thing for sure I think – is that once you progress to a certain stage, people will be very helpful, like when you are learning a language. As you do this, you can first go quickly into the water, but then move to letting yourself be seen more and more. Copy what others are doing. You have there a good environment to practice in.

I just want to add another point that I haven’t seen mentioned yet:

• In a wet suit, your body will really warm up when moving.

It strongly depends on the material and thickness, but the type (neoprene, soft-shell) that scubadivers, sailors, surfers, etc… use are truely overkill for a public swimming pool. They are meant for temperatures of at most 20°C. The heat gets uncomfortable (even unbearable) very quickly.

What might be an option for you is the one-piece bathing suits used in triathlon. They are usually thin material, and also tight-fitting, but since they go up to your shoulders, your entire torso seems connected with your hips and thighs, taking away focus from one’s mid-section.

Plus they are usually tailored in a way that makes one look more athletic than they actually are : P

Of the swimming pools I can easily find instructions for, a couple have the same kind of rules as Roesalare that @Jan-Fabry mentions. The St Niklaas rules seem helpful, since they show a picture of ‘tight’ swimming trunks with no pockets, which presumably successfully address the hygiene concern.

As mentioned in other answers, the rules against shorts should be understood as rules against normal clothing, so you can’t just jump in with the clothes you were wearing when outside the pool. But swim trunks are very common for recreational swimmers.

An example of the dresscode of a random public pool in Belgium explains what I mean (these are the allowed swimsuits):

But keep in mind that a life guard that had to deal with annoying teenagers all day can feel exhausted, and make incorrect judgements. Maybe you can have a quick chat with them before you enter the water? “Hey, I have these swim shorts, just wanted to let you know they are not regular trousers, is this ok for you?” That will get you out of the “possibly annoying teenagers” group in no time.

Here in London, loose-fitting swimming shorts – what the article you linked to calls ‘trunks’ or ‘boardshorts’ – are the norm for adult males.

Be aware that in British English – or here in London at least – ‘swimming trunks’ or ‘trunks’ means the item of clothing that the article you linked to calls ‘swim briefs’ or ‘speedos’ – i.e. budgie smugglers.

A wetsuit in a public swimming pool would be considered weird, unless you’re attending a scuba diving class.

Some people for religious reasons wear full-body swimming suits which cover all of them except their head, hands and feet, and those are fine – but I’ve never seen them on a man, only women and girls.

Your best options for maximum coverage without attracting attention are:

• boardshorts in a swimming pool, or

• wetsuit in the sea.

Remember that – certainly here in London – nobody is looking at you. Because:

a) nobody cares, and in any case

b) politeness requires that strangers ignore each other in virtually all settings.

As Willeke notes, the specific rules may vary between pools (and possibly regions), but in my experience, anything that clearly looks like swimwear, as opposed to underwear (or, worse yet, streetwear) will likely be acceptable.

If in doubt, just go to your local pool and ask. I’m sure the staff can explain what they consider acceptable swim attire. Or, assuming the swimming area is not completely hidden from view, just look around and see what everyone else is wearing.

Anyway, at least around here in Finland, the most popular style of mens’ swimwear these days (excluding loose-fitting shorts, which, as you note, many swimming pools still don’t allow¹) seems to be the body-hugging square-cut type. If those are acceptable for you, they ought to be a safe enough choice, and you’re unlikely to stand out of the crowd. But if you’d prefer to go for a full-body (or just full-torso) wetsuit, that should be OK too; many competitive swimmers do wear them, and at least they cannot be mistaken for anything but swimwear.

^{1) I remember those “no swimming in shorts” signs appearing back in the 90’s or so, when swim shorts first got popular. The main reason given at the time, like you mention, was that some people (mainly teenage males) decided that the easiest way to stay fashionable was to just go swimming in their gym / skating / etc. shorts. Since it’s kind of hard to tell shorts designed for swimming apart from other types of shorts at a glance, and since the pool staff quickly got tired of arguing with kids trying to sneak into the pool with shorts they’d been wearing all day, they found it easier to just forbid swimming in shorts altogether. Some places may have relaxed their rules since then, but some may not have.}

Different European countries, and often pools, set different rules. I live in the Netherlands and in the pools here there are no rules on what you can wear, as long as it is swimming gear and not underwear.

When I swim the competition team is also training and all of them wear tight Speedo and Adidas swimgear, male as well as females.
As that is something I am used to, I would say that all that covers you including your buttocks, tight fitting or not, will be the minimum.

The maximum is depending on the local rules.
There are many swimming trunks that are not as tight and revealing as the sport ones and will be acceptable.
I would advise you to go to a shop in the town/city you live where they know the rules of your local pool, and see what is available there.

Wetsuits might be acceptable, they are in my pool, but you will stand out and they are more revealing that many swimming trunks.

But remember, you might feel it is revealing, the others in the pool have seen worse. And most of the time we will not see your gear at all as it is hidden in the water. And not having big stretches of wet fabric will make is much nicer to be out of the water.