It is asking a lot to just be starting in modular AND to design a solid small build.
My best suggestion is to look up forum user Lugia (who is super good at rack design) and review several of his small to medium racks. Notice the balance of modules: approximate % of things that make sound, % of things that make control signal, % of other stuff that a modular needs to run. Notice the “must have” items which appear in most racks. As Jim said, there’s a base complement of utilities that are a must for most stand alone rigs. If you seriously study 5-10 of those racks it will answer a ton of your questions and put you in a better position to get a good initial plan.
That said, there’s maybe no substitute to getting in the game, making a few mistakes, and learning by trial and error what will suit you. That also argues for a bigger case; so you have room to experiment and grow. If you can afford it, two cases would be good. A tiny one to suit your small footprint goals, and a bigger one to give you some room to experiment. Or in software using VCVrack to try some of your ideas, that can help too.
Good luck!