8s are fine, especially if you have it already or want the cheapest option. Both 9 and 10 are marginal improvements in build quality, performance, battery, and they will receive updates for one or two years longer (standard is 7 years official support from google, Graphene usually supports even longer)

I'd say make selective backups of your important files, not everything since that might include the malware, and then go again through

Web installer | Install | GrapheneOS

and start over.