Quick update, got impatient and decided to just take some 70% ipa and clean the leak by the adb ports as it seems that is a common issue for soft power and it booted right up!
Will start removing the old caps this weekend, but always good to know its in a working state before you start.
Still no dice
I have noticed one bigger issue. In the Dead Mac Scrolls (Troubleshooting, flup-flup- issues, page 24) there is a description that one should be able to measure a lowish 31/30 ohm resistance over the CR20 rectifier diode. And 52/53 ohm across CR21. If I try to measure that on my board, I briefly get about the correct value but then it starts rising, and ends up somewhere over 2-2,5 kOhm. I can't quite figure out why, as I have replaced both CR20 and CR21, and the electrolytics around them.
Also adjusting R56 (+5V adj) fully counterclockwise (page 27) does not help the issue.
The Booti is the best current way to go for HD emulation. It's currently out of stock but you can get on the email notification list for when it's back in stock which should be fairly soon. You could also go with the ReActiveMicro MicroDrive/Turbo, but it's harder to get programs onto the CF card, whereas with the Booti you can just swap a USB thumb drive between the GS and your modern computer to get stuff on there.
I see now you said you had a mouse freeze, meaning you can’t move the mouse at all right? Is this happening every time? Often that indicates a hardware-level issue and Macsbug might not work. (In that case, almost nothing will work really ... ). I had similar issues to what you describe with a Turbo040 installed in one of my SE/30s in the past, never could figure it out entirely — seemed to be heat related (after a while the thing would run hot, and issues would occur randomly).
But yes to activate Macsbug you should just be able to hit the interrupt switch, assuming one is installed. If Macsbug isn’t installed, that should bring up MicroBug. Either way, if nothing happens, the Mac is past the point of diagnosis and must be reset.
If you CAN get Macsbug up, the next question will be to see what chunk of code is running at the time of the crash. “WH PC” may tell you, or if not do a stack crawl with “SC6”. The Macsbug reference linked above describes these commands - I actually think it’s one of the best Apple technical docs of the time, with great examples and even a basic overview of 68k assembly, but it does require patience to learn.