Thanks for the advice, everyone. I got it (champion edition from highscoresaves, ordered mid-March) put together, but it boots to a solid dark green screen with nothing but the backlight on the LCD. I've troubleshot and I don't see any issues.
EXC5 may need to be temporarily shorted to remove keys that not been flushed
I shorted EXC5 last weekend after removing the battery a few days before. On the underside of the board, I used a wire to short the 3 pairs of EXC5 pins, then held one end of the wire across two of the pins and connected the other end to the third to short all 3. I held it for over a minute and did this multiple times in a row. I'll do it again when I open it back up.
Broken pins on multi kit
I don't see any. The big board appears to be pushed down as far as it can go, and I think I got the smaller board in far enough. Pictures of both attached.
Interconnect board not installed the correct way
Up arrow is facing up, and I pushed the big board in from the bottom, appears to be a snug connection. See pictures.
Key writing wires are not installed correctly
I re-did the red wire, it doesn't look great but I think it's okay. Tested continuity between each pin and the surrounding pins and didn't find anything, continuity between the pins and the harness ends all checked out. Nothing appears to be touching. The order should be correct (see pictures). I initially tried with the resistor connected, then disconnected it when the first attempts didn't work. Solid green each time.
Correct firmware has not been installed onto multi kit
I haven't messed with the firmware.
Correct ROM pack is not installed on the SD card
The link in the PDF is dead, but I found Darksoft CPS-2 2020-05-24.7z elsewhere. I unzipped it to get CPS2 Roll-Up Base Pack. I formatted my SD card and put games in the root, then each game directory (19XX is the first) in games. Each contains flash.01, flash.02, flash.03, flash.05, flash.07, flash.09, flash.11, key, and NAME.
SD card is bad
It's an 8GB card that came with the kit from highscoresaves, and I connected it to my PC again and all the files appear to be intact.
What should I try next? Thanks!