3rd gen X-series had a pretty sensitive OPP profile which is also seen on the first revision of Seasonic Focus. You have a 130W CPU which eats ~300W at 4.3GHz, a GPU which draws some +12V current spike around 55-60A(not sure how much the EVGA draws) and lots of other stuff which take double-digit to 100 watts total. Going with most "modern" mainstream 850W models (adapted to RTX2080Ti) will solve the problem, but I won't call a 1000W PSU an "overkill" for this build.
RMA came through, got a Focus GX-850 and it hasn't crashed after pushing it. Thanks for the recommendation.
No problem
The Seasonic (OneSeasonic) Prime PX-1000 PSU has absolutely no problems with a *shunt modded* RTX 3090 pushing 566W (!!!) in Overwatch at 4k. The newest lines are perfectly fine.
However the old "X series" and probably the SS-1000XP and 1050XP series are not fine. Saw someone with a X-1050 have the system reboot on their 3090.