Good luck with this. I'm a modular newbie (so not a lot of direct advice to give on modular) but a longtime musician. I'll be interested to see where you land with this inquiry and build, as I'm in a somewhat similar position.
I am curious, what are you hoping to get from modular that you can't from software & standalone hardware?
Hearing about your "retro" focus, I would think you could be very well served with some non-modular options such as:
-- Roland software, Roland emulations, Roland hardware (like Jupiter Xm). IMO this stuff sounds great and is a joy to use. If I worked in retro like you do, I would definitely have an "Ultimate" Roland Cloud subscription
-- Sequential's current lineup: Dave Smith is still making beautiful instruments that mix the classic vibe (and presets) with modern capabilities. Any of those analogue boards could give you a real boost in great retro sound
-- less completely retro but very awesome is Native Instruments Komplete + their Komplete controllers. The hands-on integration for these is now very strong, and the NI library is so vast. I love the browsing and sound manipulation that can be done here without a mouse now. IMO this is also a great value, compared to equivalent spend on hardware / modular.
-- the Arturia software suite is also great for retro. The sheer # of retro instruments and presets is huge. The "modular" VSTs in that suite could give you a lot of the retro modular sounds, if that's what you're after.
I assume you know about most if not all of those above already. BUT, hearing your situation and objectives, I thought I would mention these as possible alternatives for you; to me they really stand out as potentially useful for your situation.
Having recently entered the modular domain, I can say: i) it is expensive ii) its very deep in terms of variety of modules, module designs, etc. iii) there's a vast amount of learning to be able to buy or run a decent rig iv) it does beat software and standalone synths for some uses, but not all. I AM enjoying modular learning and sounds a lot, and it also helps me understand and me appreciate my VSTs and hardware drum machines even more!
Best wishes,
Nicholas