LS-120 was available in a SCSI form, but in reality, it might actually be even easier to use both LS-120 and ZIp250 with USB on a beige PCI Mac than to bother with IDE. I believe there are also SCSI Zip250 drives, probably both internal and external, so that's also an option. Plus, that upper bay is 5.25, so you're talking about room for all manner of things, really.
If the IDE was "bad" (I'm betting it's not) you could even put a SCSI2SD v6 or a regular period SCSI hard disk in that space. Though, CF or a fast-ish SCSI hard disk (there's probably enough room in that bay for a fast disk even though the bus isn't that fast) for video capture is a thing.
The other-other thing is, for a home prosumer video capture setup, the Avid Cinema card uses a PCI slot, and is supposed to have worked with the original stock hard disk. So, just upgrading that disk or, say, moving your boot disk to SCSI and adding a bigger CF card or bigger/faster IDE disk for video capture and scratch might be worth looking into. Then, you have Avid Cinema in one PCI slot, USB in another, and CommSlot2 Ethernet.
How great? The 6500's 2-meg Rage II will do 24-bit at up to 800x600 and 16-bit up to 1152x870, which is pretty normal for ~1997. I don't recall if VRAM is upgradeable as it is on cards like the XClaim VR or on the beige G3 and iMac motherboards.