I am not that experienced myself, so please keep that in mind when considering my answer!
For that very same reason, I may give a good advice, though.
I think the Plonk and the drum sequencer sounds like a good thing to get! A versatile generator of percussive sounds and a comprehensive sequencer to drive it (that still has an (seemingly, to me) intuitive and solid interface.
Do you need to buy all of them? To me the Atlantis and Rainmaker appears to be interesting modules but perhaps to much functionality put into one module for someone who is just beginning to learn. Also with what looks like alot of menues. I may be wrong here.
I think it is sometimes not a good idea to get a do-it-all thing - in many contexts, not just synthesizers - to begin with, even though it seems like you get alot all at once. Yes, in a way - but it may prove too complicated and just be sitting there not used.
Both the Atlantis and Pico are complete synthesizers in themselves, I think. Perhaps one of those would be enough to learn to begin with. The Pico, if I understand correctly, is additive so it may be a bit of a different approach. You may find it interesting, though.
Also, I notice that most of the modules are very big. Do you have space for them? Could the space be used differently? How will you organize it?
Perhaps get smaller parts with clear functions and learn how they interact with each other. Then the understanding of the multi-module will come automatically (as well as ideas of how to use it). Or learn it through perhaps one big module and a few small ones.
Apart from intellijel (where I have been happy with every module I’ve tried) you my want to look into Dreadbox’ Chromatic modules (none of which I’ve tried). I have heard they consist of solid quality, smart design and for a very fair price. Not unlike Intellijel in this matter.
I will repeat: I am not an expert and do not have comprehensive experience. And I do not follow my own advice (which is a decision I am quite happy with)!