Your title is about backups but your question seems mostly just about how to set up your storage for backups.
You can go about pooling disks in a few ways but you first need to define what level of protection from failure you want. Before going further though, how much space do you project that you will need for backups?
Look at the mistake. Ask yourself why you made the decisions that led to the mistake. Ask yourself what should you have done to avoid making that mistake. Remember this and don't make the same mistake again.
If you want simple you'll have to manually decrypt each time it needs doing.
If you want it to be "automatic" then your best bet is something network based. A "simple" would be to just have a script ssh's somewhere, pulls the decryption key, and then decrypts the disks. There's plenty of flaws with this though as while a threat actor couldn't swipe a single encrypted disk they could just log in as root, get your script, and pull the decryption key themselves.
The optimal solution would be to also encrypt the root partition but now you need to do network based decryption at boot which adds further complexity. I've previously used Clevis and Tang to do this.
I personally don'tencrypt my server root and only encrypt my data disks. Then ssh in on a reboot or power event and manually decrypt. It is the simplest and most secure option.
I backed this: https://www.crowdsupply.com/cool-tech-zone/tangara
Its not yet released and is in the manufacturing process but I think it's worth considering
They do not work well in already humid environments but in a hot and dry climate they do quite well. It absolutely does add to the humidity (obviously) but speaking from experience I'd rather have a evap cooler than not if my AC is out.
The largest difference in utilizing one over AC is that they rely on airflow so you need to actually ventilate the area you are cooling as compared to AC where you want a sealed space.
The main driver is power efficiency. Only thing they are doing is running a small water pump and a big fan.
The domain doesnt really matter. The mail servers reputation is what really matters. If you aren't going to run your own mail server then you have nothing to be concerned about.
Debian
Sounds like my usual load. Flamethrower, shield pack, then OPS and 500kg depending on planet conditions.
If there is orbital scatter then I'll leave the OPS and take something else.
I take both!
They watch the swarm. You can view connections of the swarm. They see your ISP owned IP and send an email.
Debian
If the total data is 3tb and you want disk failure protection I would take your two 6tb disks and put them in a mirror. With the amount of data you have and the drive sizes at your disposal that makes the most sense. This leaves you with 3tb free for growth. If you wanted an additional backup I would recommend storing it in a different location entirely or pay a cloud provider like Backblaze.
I would do this with ZFS but you can also do this via LVM or just straight md-raid/mdadm. I'm not sure what your issues are with zfs on popos but they should be resolvable as Ubuntu supports zfs fine to my knowledge.
An alternative you could consider is using mergersfs to logically pool indivial filesystems on each of the disks and then use SnapRAID to provider some level of protection. You'll have to look into that further if interests you as I don't have to much info in my head related to that solution. Its not as safe as a mirror but its better than nothing.