I don't have an iBook G3, so I can't offer any tested solutions.
If you're on Yaboot, what happens if you append with 'radeon.agpmode=-1', like at the boot prompt, 'Linux radeon.agpmode=-1'?
Failing that, and aside from not using the non-free Radeon driver (which is not a very good option), I would just revert back to Debian 8. I've heard it works well with iBooks. It's still got Arctic Fox, was last updated in 2018, and should respond faster due to the (probably) lighterweight kernel and userspace, especially with Xfce.
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I originally tried 8, and it has never worked properly. I was hoping that the problem with Radeon would have been fixed by now. Is it still under development?