Okay. I have updates.
I bought a couple of special nuggets on eBay. I got a virtually unused (they weren't kidding, there wasn't a speck of dust inside it) LaCie FireWire 400 External drive. I also managed to find for reasonable money a later model IDE Panasonic DVD+RW/DVD-RAM drive that takes bare discs and cartridges (which is relatively rare on later drives). I did some swaperoo and now have a Firewire 400 DVD-RAM drive. It works effortlessly with no drivers or extensions needed (as far as I can tell) in Mac OS 9.2. So I guess my original issue was just that the SCSI drive wasn't recognised by Mac OS 9 unless it had a little help. I think I'll try the CD/DVD hack mentioned earlier and see if I can get it working without DVD-RAM Tune-up extension.
I also have a bunch of 5.2 and 9.4 GB double-sided cartridges to goof about with.
didnt you have to "format" dvd-rams?
Yes, and I had formatted the disk, I think the problem I was facing was I formatted both sides of my Test disk in Mac OS X as FAT volumes and OS 9.2 doesn't support using FAT on DVD-RAM (or at least I don't have it working, I wasn't offered the option when initialising the disks in the Firewire drive either). I was able to Format them as HFS volumes and they work okay. Mac OS 9 also support using UDF which might allow me to use the disks cross-platform (with usual destruction of Resource Forks caveats applying).
Also I watched that Technology Connections video, it was a very interesting deep dive.
FAT32 works finen with USB sticks
It does and I have used FAT32 USB sticks in Mac OS 9 before just fine. I guess maybe Apple just didn't want you to use it on DVD-RAM or something (because they offer UDF for cross-platform). Who the heck knows.