I think it’s critical to know what you want from Modular. Not having a plan will lead to the aforementioned unfulfilling & frustrating experiences. Like many I bought an 0-coast and quickly sold it and got a case and started spending.
Luckily for me I read this piece of advice somewhere early on which stopped me replicating things my desktop gear does.
Modular to me is for experimentation, ‘what happens if I put that there’ (can’t do this with desktop synths, well most of them anyway), weird oscillators & sound generators, esoteric sequencers and modulators - anything unusual is the aim. If I want to use the sounds in any tracks, the case can be sent into the OT. If I want to just play with it and see what sounds come out then it’s ready to go. If I want to spend a month developing a complex patch I can.
I won’t say I haven’t spent lots of money I shouldn’t on this - I don’t need it at all. But this one piece of advice has kept it in the right area for me, and so I’ve been able to see it as a (musically) justifiable addition to the rest of the gear.
Footnote: I re-bought the 0-coast, it adds functions to my main case and provides a travel/garden/holiday patching fix, and it’s really, really deep.