> doubt between making a ZFS mirror, making 2 independent pools

You will gain protection agains bit-rot and self-healing (via scrubs) with a mirror. Also faster reads.

> mix more older HDDs

You can do this with ZFS too! As long as you have two HDDs of the same size (or similar size, as to not loose too much to unused space), you can also add them as a mirrored zdev to your existing pool (or make an new one for backups as you wrote). Only the two disks in a mirror need to be of similar size, not all disks in a pool.