FYI.... status update....
Found an issue with the modified circuit board I made. I had the "ACK" and "ACT" lines swapped. Oops! This was easy enough to fix with a cut/jump on the board though.
I was able to get the RaSCSI software running on a Raspberry Pi 4. My Quadra is able to see the virtual hard disk I created, as well as format, read and write to it. I ran a quick benchmark, and the performance seems to be on par with my SCSI2SD v5.
My current sticking point is with CD-ROMs. Using various SCSI tools, the Mac can see that there is a drive present at the correct SCSI ID, but it hasn't detected any of the images that i have "Inserted". So, I'm going to keep working that.
In the next few days, I'm hoping to run some benchmarks comparing the different versions of Rasperry Pis, and maybe on some different Mac models. But first, I'm trying to get the CD-ROM functionality working.