Generally, I'd keep G3s on OS X for a couple of reasons, including for their typical GPU support.
You could slap in an SSD and make a nice, optimized Tiger build with the TigerSierra theme. Give the thing a specific purpose, like undistracted writing.
Maybe a stupid suggestion, but a little while after Buster releases on the 7th and Bullseye's cycle has been up and running, you could try your luck with Sid again. Hell, you could even bring the problem up on the mailing list and see what the devs have to say. They could end up investigating it.
Worth a shot.