As I need a working AND portable system ASAP, I decided to postpone further fumbling around with the annoying situation (both 280 AIO and 240 AIO vs PSU vs 2nd m2 slot) and pick the "FU edition" aka S400 V2 w/ PCIe 4 riser + AXP120-x67 + Silverstone Air Slimmer.
New specs (2024-09-22)...