score:7
Beyond preparing documentation of the relationship - do I need to be aware of anything else?
Not really. You won't be spending enough time in Canada to have to worry about becoming a tax resident (such that they would hold you liable for non-Canadian income). I don't know how that works in Canada specifically, but in the US and other countries it starts to be a problem if you spend more than half of your time in the jurisdiction.
Are there special rules I need to be aware of given the frequency of visits?
Not really. It sounds like you're already aware of the fact that this pattern could make it seem like you are trying to live in Canada through successive visits. But as you are probably also aware, there are lots of people who travel frequently between the countries, it should be obvious to the CBSA/ASFC that you're spending a fairly small proportion of your time in Canada, and the more you continue to go back and forth the less likely they will be to suspect on any one trip that you're planning to stay or otherwise violate your conditions of admission.
If your job allows you to work remotely, you'll probably want to know that Canada is unusual in explicitly allowing people to work remotely while visiting Canada. It is the only country that I'm aware of that permits this explicitly. Note that this concerns immigration law. I do not know whether this would cause liability to arise under Canadian tax law (for example, because the work would be performed in Canada), nor whether your employer's obligations under US labor or tax law might lead them to forbid or limit this.