> Morph.zone's library reports that the 20-inch iMac iSight is an
> “unofficially supported machine since 3.12” and the 17-inch iMac iSight
> is an “unofficially supported machine since 3.16”.

That page (last updated with MorphOS 3.16 release in March 2022) is out of date since MorphOS 3.18 release in May 2023 which added official support for both iMac G5 iSight models.

> MorphOS's official website lists the ATi Radeon X600 XT
> as a supported GPU, not the X600 Pro

I doubt this is the reason for your bad experience. After all, the 17-inch model is officially supported (see above). So the actual machine(s) the MorphOS developers or beta testers had at their disposal to declare that model as officially supported must have had the same GPU as yours.