Motherboard standoff screws are machine screws, so how do I fasten those to wood securely? Can I forgo standoffs entirely since wood isn't conductive?
Also, I'm looking at a spare Radeon RX 5600 right now and I can't find any holes that would make suitable mounting points (RX 7900 XTX is what I actually plan to mount).
I see an L-bracket under the PSU so at least that one looks straightforward at least.
Edit: It looks like that PCIe ribbon comes with a mounting bracket attached. Maybe that's how they did it here.
Just wood screws. You don't need inserts unless you're looking to make it more complicated. Pilot-hole first, then screw. You don't need to worry about earthing the board.
I've been running motherboards on plywood for over a decade. (Actually, much longer. I built a plywood case back in the 90s) Okay, more horizontal than this and less aesthetically pleasing, but that's because I put them in a noiseproofed cupboard. The only real tricky bit is supporting a heavy GPU if you have one. Otherwise it is simply a case of fixing them to a piece of wood so they can't move around.
Oh, and keep cats away.
Mounting the GPU securely is currently my greatest concern.
The cats problem crossed my mind. I have a big CPU cooler assembly which would make a tempting ledge for cats to jump and perch on. I've considered putting a plexiglass covering over it for that reason.
I built a little bracket for mine to stop the GPU flapping around.
Here's a pic, which explains a bit better: https://imgur.com/tCtpH8g
Oh so yours is directly plugged into the motherboard. I plan to use a ribbon cable like my example picture.