Move pool to new HDDs - Best Option ElectricEel

I would suggest against running 3x 12TB drives in RAIDZ1.

I suggest reading iX's ZFS Pool Layout White Paper and Assessing the Potential for Data Loss | TrueNAS Community.

Fresh install is good. Do note that SCALE requires an SSD as a boot drive.

Correct.

More or less.

Afaik, yes. Things might change in the future.