Does Bazzite have good Nvidia support and do they have a good track record with testing updates? I don’t want my graphics card to suddenly stop working after an update as it likes to on Kubuntu.

Yes, although nobody is perfect so it's always a possibility. The difference with Bazzite (and other immutable distros) is if it does happen, all you need to do is reboot and choose the previous version in grub. It'll be like the update never happened, no need to look up or run any magic commands.

Then you just wait until they fix it before updating again.