Not much, as it only works on very few high end phones not sold in most countries. Hopefully their Motorola partnership will expand its availability but I'm not confident that'll happen anytime soon.

Ideally yes, otherwise any other AOSP-based ROM. There are many, and they support far more devices than Graphene, though implementations of e.g. Google Play services is more hacky.

The most well-known: https://wiki.lineageos.org/devices/

Yes, but there are issues.

You can’t use stuff like banking apps on a modified device and losing access to normal android devices would be a big blow to the momentum of the F-Droid community. GrapheneOS might not be a big enough community to sustain work on the projects delivered by F-Droid.