As above - from experience I'd say forget the SATA bridge idea.
It's slightly off at a tangent, but I've had similar problems with a PVR box with a PPC 405 CPU. It's fine with any ATA spinning rust. Ideally I wanted it near silent so tried a couple of SATA to ATA bridge adaptors that I know are...
As Unknown-K mentioned earlier if you wanted a high res 2D card for classic Mac OS you went to a Mac specialist graphics board maker like Radius, IxMicro. I think the Matrox Millennium worked as well.
At the end of that era ATI Rage, 3Dfx and Riva TNTs etc were gaming cards, not general purpose...
Supernova, I can't help but think you're judging the late 90s world of PowerMacs by 2017 hobbyist standards. Sites like this didn't exist, closest you would likely find be a usenet group. People were mainly using the machines in period to do real work, not as a hobbyistproject. My employers at...