Just remembered seeing this thing, and it appears to be thin ITX. Two different AMD Ryzen embedded APU’s to choose from with a built in Arduino:
Also, as far as getting the standoffs level, there are a few options. On one case I stuffed a board into an old VCR and the bottom was uneven with different levels to it. I started at the highest spot and put a standoff through a penny and glued it down, then proceeded to do the same with the rest. I just stacked up pennies and glued as needed to get the height adjusted. I used some extensions for some of the rear coastline I/O and didn’t need to get that lined up. You might be able to get some aluminum machined to thickness and epoxy it in place as a better solution.
I’m sure there are a few ways of going about it where you get something to the right thickness and glue or epoxy it down. Possibly even a thin tray to mount the board on that has just the right thickness and can be slid in and screwed or latched down.