I do like a success story. Making these old systems work with modern components can be challenging. But when you do make it work it's so rewarding. (y)
I have one of those sitting in the attic, the power mac, not the imac . . ;)
From my limited experience, it's the usual suspects . . PSU and leaky caps.
If it has an airport card, they usually crap out too.
Great video, Tons of information. I use a CH model similar to yours, except without the top button on the stick. I have an original Apple, but not working. Running my card on an LC475. I picked up back in the 90's for $5.00, recycle rescue. Bought my 2e card on eBay around that time. Brand new...