Lets start with me - I'd go down one of the following roads - all of these being workstation-focused:
1. Custom 11 - 10L mATX SFF case with an mITX option:
12900KS, MSI MAG Z690M Mortar WIFI, 280 mm radiator (or maybe even 360), 64 GB RAM, 2 x 4 TB SSD (FireCuda 530), RTX A4000 or A4500 with RTX 30xx Heatsink / Cooler mod (or just plain Liquid Cooling), Cooler Master SFF 850W PSU (or just use a custom 1U server PSU with similar to FlexATX dimenions, with at least 1000 W and 80 Plus Platinum or better).
Oh, and keep it portable, too, so something like a steel skeleton with either aluminium or even just PVC or similar side panels.
Would also work nice with a 200 x 200 x 45 radiator.
Rough dimensions: 13 * 26 * 32
2. 2-in-1 System in the Meshlicious or similarly sized case:
mITX main system + mini-PC as a sidekick, eg. a Ryzen 9 5950X (ASUS ROG Strix X570-I Gaming) or a 12900K (MSI Z690), rest of the specs very similar to the one system detailed in #1, plus an internal GPU output option to be able to quickly switch between both; something on the lines of a custom KVM switch.
The mini PC would be there for when I dont the raw power of the main system, eg. when going to bed and wanting to listen to music, light browsing and other stuff, so even a Raspberry Pi would do.
3. Rugged Notebook SFF system: Something on the lines of a 12700k or 5950X, but in a sub / around 5L case, similar to a notebook, eg. with a mechanical keyboard, (low profile / ThinkPad notebook <= Tx20 generation), screen (FHD) etc. Fun with heatpipes and small-sized radiators, wind channels etc.
cu, w0lf.