What route of transportation between Oxford and Amsterdam optimizes economy and convenience?

I've done the Oxford <-> Netherlands trip quite a few times over the years. Firstly, I'd say you're missing a few more options:

• Bus to London, Eurolines or iDBus over to Europe, possibly direct to Amsterdam or possibly changing to another bus/train
• Bus or train to one of Birmingham Airport, Luton, Stansted or Marston (Kent) for a cheap flight
• Train to London, Eurostar to Brussels, then either local train (cheaper, book "Any Dutch Station") or Thalys (quicker) onwards

Ignoring costs, I'd say the easiest two options are:

• Airline bus to Heathrow (esp. T5), flight to Amsterdam, train from Schipol to the city centre
• Train to London, short underground ride, Eurostar to Brussels, Thalys onwards to Amsterdam

The former might be slightly quicker, but the latter gives you several 1-2 hour periods where you can get useful work done / relax + read a book / etc.

For trips when I needed to spend the whole working day in the Netherlands, then the Dutch Flyer was my go-to travel method. Leave Oxford after work, train into London (Paddington), 5 minute walk to Lancaster Gate, tube to Liverpool Street, quick meal in one of the restaurants near there (often good deals available online!), later train to Harwich, up the escalators to checkin, onto the boat, and in bed at a sensible time. I'd describe the ferry as a "floating travelodge" - basic accommodation, but effective and decent quality. In the morning, decent shower, down the ramp into the station, onto a waiting train, and short but picturesque ride along the coast to Schiedam or Rotterdam. Grab some breakfast as you change trains, then off to your final destination. Surprisingly easy, and very effective! Coming back overnight, there's a decent chippie at the ferry terminal in Hooke, just past the end of the platform, and you can normally make it onto the ferry and sit out on the deck to eat with your food still warm! It's an early arrival into Harwich, but you get into London well in time for a full day's work, and even back to Oxford for only a slightly late start. (Never done daytime, sorry)

One other thing - there's no "universally right" option. Almost all of the modes of transport available operate on "airline" style pricing, where costs vary, and typically increase closer to departure. You'll need to check all of them - flights from many airports, trains, buses etc.... Also, consider booking horizons. Eurostar used to be ~4 months, but now it's a little longer but released in chunks. Eurostar + Any Dutch Station will work like that, but Eurostar + Thalys would be constrained by the Thalys booking horizon, which is ~3 months. I think Eurostar + Any Dutch Station starts from £89 return, though it can be tricky to book at times, and you might need to phone / use a different website.

For both Eurostar or Dutch Flyer, you've four options for getting to London. One is bus, with the risks of delays and missed connections that go with that, but it can be cheaper. Another is normal train tickets, which might be cheaper if it's off peak both ways. Next, you can buy cheap advanced purchase train tickets to London if you book far enough out, but there are connection risks there (they start from £7.20 one way). Finally, I normally buy a "Euro High Saver CIV" ticket, which is currently £31.50 return with a railcard. It's valid on any train any time of day (railcard discount permitting), includes the cross London transfer, extends the CIV protections (including delay rebooking) back to Oxford, and is valid for connecting to Eurostar or a rail-sail ticket from London (eg the Dutch Flyer)