In order to begin taking prep-tests and not just studying, what's the best way to begin?
Should I start with taking timed tests (as I've been going through untimed practice questions for a month)?
Or should I start with untimed tests?
Somewhere in between?

Advice please!

I would say, take at least one or two untimed tests to start out with to make sure your accuracy is at or near 100%. Then move on to timed to see which sections you need to get faster in, and drill those sections.

Whatever you choose I would recommend the first thing you take is a full diagnostic, timed (and timed breaks), with a proxy experimental section. This will get your bearings straight and you can plan studying accordingly.

also, though I took untimed sections over the course of my studying, an entire untimed test seems weird to me, and not that much fun at all. Again, after the first diagnostic you will see if time is really an issue. If you ended each section without having to random bubble, then do every test timed imo. If you had to random bubble, then maybe either do some untimed or some with 40 minutes, and work on speed. In general, it is not advised here to do too much of any PTing untimed. It's not realistic and would a horrible habit to get into. If you choose to do some that way, make it 2 or 3 at most.

+1