ZFS doesn’t really support mismatched disks. In OP’s case it would behave as if it was 4x 2TB disks, making 4 TB of raw storage unusable, with 1 disk of parity that would yield 6TB of usable storage. In the future the 2x 2TB disks could be swapped with 4 TB disks, and then ZFS would make use of all the storage, yielding 12 TB of usable storage.
BTRFS handles mismatched disks just fine, however it’s RAID5 and RAID6 modes are still partially broken. RAID1 works fine, but results in half the storage being used for parity, so this would again yield a total of 6TB usable with the current disks.
I really don’t see much benefit to running two clusters.
I’m also running single clusters with multiple ingress controllers both at home and at work.
If you are concerned with blast radius, you should probably first look into setting up Network Policies to ensure that pods can’t talk to things they shouldn’t.
There is of course still the risk of something escaping the container, but the risk is rather low in comparison. There are options out there for hardening the container runtime further.
You might also look into adding things that can monitor the cluster for intrusions or prevent them. Stuff like running CrowdSec on your ingresses, and using Falco to watch for various malicious behaviour.