Interesting thread!
For me, top-down works better than bottom-up. Here is what that means:
In a certain sense, I used my existing vanilla synths and fx as a modular system (on a different level) so far. When stacking synths, that´s similar to combining OSCs. Likewise, building processing chains like (Synth A open filter) -> (delay) -> (Synth B audio in) -> (pedals) and so on. I´d call this quarter-modular in contrast to semi-modular. Or quadrant-modular if you want to be a smart ass :D.
Next stop: Semi-modulars. Opening up the box-internal signal chains at some points, granting more access. A good point to get started and learn, as you don´t have to do everything from scratch, but can gradually grow into things. Just inserting a filter or adding that 2nd and 3rd LFO is pretty easy this way. Buying and learning on an "as needed" basis is more fun to me, as I get instant results I can use in my tracks.
And then of course, the next step is full modular, a.k.a. crack effect.