Well, than, you should bge fine with VMUG subscription to get licenses. The options you have mentioned, should be able to run it.
snatch1e
So, yeah, the most decent option is unRAID.
It will handle different size drives (the biggest drive will be used for parity).
As it was mentioned, there is no real need for SSD caching unless it is required by your workload. In any case, you should be able to configure caching later if the performance of HDDs wouldn't satisfy your needs.
Basically, I don't think there is a way to check that. Usually, with the new drive you get a warranty, so even if it fails in some time you get the replacement.
Do not forget about backups since even healthy drives can fail at any moment for any reason, so you have to be prepared for that.
You may get additional server and run 2 node cluster with Starwinds vsan. It is hardware agnostic, so, basically, hardware won't be an issue, also it doesn't require witness node which might save you from getting 3rd computer. I believe it is the most decent option for 2-3 node setups.
It really depends on your needs. If you can go with 20TB over 22TB and it won't create any issues than better deal for 20tb drive makes more sense.
Try to connect the drives in another machine without enclosure if possible.
It is normal if you have the encrypted drive. If you want it to show up normally without password on any machibe than remove encryption.
I do not think, that this rate will make much sense for users since it will be visible with the huge amount of drives. So, I will still look for the best deals on the drives rather than brand.
For the mixed size drives, I believe, one of the best options are unRAID and mergerfs with snapraid. Also, if you want to use windows, stablebit drivepool would make it and snapraid can be used for redundancy.
Get an HDD for it. Internal or external, depends on you.
Alternatively, you may store it in Backblaze personal.
So, basically, try to follow 3-2-1 backup rule to keep your data safe. https://www.veeam.com/blog/321-backup-rule.html
The most default configuration is a local backup (on external drives or backup NAS) and a cloud copy. Cloud storage can be pretty costly for much storage, so at least have it for the most critical data.
Also, keep in mind that https://www.raidisnotabackup.com/ , but it's obviously good to have it for better availability of the data.