2018 Mac mini won't turn on

First thing's first. I would give the logic board a thorough once over. Check and see if anything just looks bad. Feel free to add some pictures to your original post.

I assume by the fact that you're not getting it to detect in DFU mode, that it's not powering in at all. This is a board I don't have a ton of experience with honestly, but based on the schematic, there are some debugging LEDs that were likely present during production, that probably aren't on the actual board. They are similar to the LEDs used in an iMac for diagnostic purposes. You man be able to make use of the pads, regardless of the presence of the components, but I want to know what's actually there first, since I cannot tell from this picture. They're located along the edge of the board, by the RAM modules.

EDIT:

Based on further looking, I suspect all this stuff is blank. Because all the mosfets on the bottom, looked blank. But, those could likely still be used to check vital system rails. You would just have to use the mosfet pads, since that's what would let current through to the LEDS. That aren't there. Some of these also feed into each other. But you can check for incoming voltages, or shorts to ground on the mosfets since the traces that feed them will still be there.