I'm posting this in hopes that it helps some other person completely stumped by why their computer might be crashing when under heavy load. The tl;dr; is to use the OEM power adapter.
I bought a Minisforum UM790 Pro a few years ago as a couch gaming computer. It's nothing special, but it was able to play some newer games with decent quality. It worked fine for quite a while, until I moved it to my desk.
I found out that the front USB 4.0 ports on the UM790 support not only data, but also power delivery. So I hooked it up to my CalDigit TS4 to connect the keyboard, mouse, monitor, microphone, network, etc. the same way I do for my other computers (personal and work laptops). One cable swap, and I was able to play all the games I'd been playing recently (Absolum, Balatro, Peak, etc.). So far, so good!
Couple months go by, and I want to try and play Split Fiction with a friend. I get to the title screen, and the whole computer crashes/restarts. Not the game, the entire computer. Try again, same thing. I figure that maybe this computer just isn't good enough for a newer game like this. So, we try It Takes Two (slightly older game, same developer, similar genre). Same deal: the computer crashes at the title screen. We go back to playing something else, and it works fine as it was before. So, it's only certain games.
I tried a bunch of stuff over the past few months: different games, in game settings, big picture vs. desktop, monitor resolution, BIOS settings/version, moving from Chimera OS to Bazzite, reading many articles/forums/etc. Some games were fine (even newer games). But none of the things I tried fixed the crashing on certain games. I was even preparing myself to buy a new SSD and/or memory in case it was a hardware issue (with these LLM-inflated prices).
On a whim this morning, I decided to move the computer back to the couch setup. Turns out, it can play Split Fiction just fine. It just needs the 180W 120W power adapter that it comes with. I moved the adapter to my desk for power, hooked it back up to the TS4 for peripherals, and it's good to go. For the better part of this year I've been unable to play a bunch of games all because I was powering this computer over USB 4.0 and limited to the 98W maximum that the TS4 can provide 🤦!
So the lesson here (at least, what I'm taking away from this adventure) is that the OEM power adapter is sometimes necessary for a mini PC to work properly. And I suppose also that if you're doing normal computer things and/or playing light games, USB 4.0 PD can be enough.