Hi Logical Phallusy,
On one hand it's interesting to read your explanations (a bit like myself) on the other hand it's difficult to judge on what/where you need advice, input and/or feedback. Perhaps the best is that you summarise it in two maximum three lines and then below that mention the questions you have? I haven't come across any real question or did I overlook the questions of yours?
Some general information: I would add one extra row to the rack and keep that empty for adding modules in the (near) future. You still can stay within your 4k budget (going to be tough though) but you have at least the space to extend in case you need, and you will need that :-)
You mention the Rubicon 2 from Intellijel is expensive and indeed that's the case. I got this one myself and though it's certainly a good module but is it worth that kind of money? I am not sure about that, for less money and at least as much fun I enjoy my Make Noise - STO. Perhaps to keep in mind that larger/bigger/more expensive VCO/DCO modules doesn't mean automatically more fun to play with. Of course it depends a bit on what you want to do but your observation is right that the Rubicon 2 is (bloody) expensive and you might want to ask yourself if that's worth it for what it gives you. Best advice is to have it tested at your local dealer. Testing the modules before buying is the best what you can do; not always possible though.
Also generally I would advice to start with a simple setup of: 2 VCOs, 2 LFOs, 2 EGs, 2 filters, ring mod perhaps, noise?, VCAs, mixer and audio input/output module, that as a contrast to your choice of "quite sexy modules" ;-)
Good luck with your planning, as you mentioned it yourself already, start slowly with a few modules and build it up bit by bit, while gaining experience, that's indeed the most enjoyable way into modular. Have fun and kind regards, Garfield Modular.
For review reports of Eurorack modules, please refer to https://garfieldmodular.net/ for PDF formatted downloads