As already stated, yes Backblaze B2 is only for data. You cannot ‘install’ anything to it. B2 is Backblaze’s competing product for Amazon’s S3. Note, however, that you will have to use Minio as a bridge between Seafile and B2 and there are caveats with that approach. While B2 is much cheaper, it may be better (given your small data-set size) to just bite the bullet and use S3 since it is directly supported by Seafile without needing any intermediaries. If you go the B2 route, feel free to message here or contact me directly and I can help you with setting up the correct Minio container for B2. I made a few posts about this in the forum last year so you can search for those too.
Regarding the whole JBOD vs RAID thing. JBOD means you are just using a bunch of disks with no redundancy. That is fine if you have solid backups but any failure of a disk means you have to replace that disk and restore from backups. You do not have the option to run ‘degraded’ like with RAID. If you use something like S3 or B2, this is entirely irrelevant since you are not running any ‘disks’ but are instead using the cloud, does that make sense?
Meaning no offence, I would suggest against JBOD as an operating mode. I’m not advocating RAID necessarily, I’m just saying that if you are going the non-RAID route, simply connect your disks normally and use regular SATA AHCI mode. Switching the controler to JBOD means you are using the software RAID but asking it to not do RAID. In that case, why use the software-RAID at all? Just let the native SATA controller do it’s job directly. Faster, easier and no vendor-lock-in.
Regarding ZFS: This is like the ‘gold standard’ of data storage. It’s awesome. It’s also expensive and hard to set up unless you know what your are doing. You absolutely require ECC RAM and a server-class motherboard. Don’t try to do it on a ‘consumer’ or ‘gaming’ motherboard. This greatly increases your expense for no real benefit in your situtation, IMHO. If you have the money and experience then totally do ZFS! It’s amazing and I will never knock it. If you don’t have the money and experience, you will likely screw it up and regret this life choice – not knocking the suggestion, but just saying that for a first deployment you may want to err on the side of simplicity. Down the road as your needs expand and your experience grows, you can consider ZFS implementations. That’s just my opinion, however.
As @gogofc mentioned, use a regular Linux distro vs TrueNAS for exactly the points he brought up, he’s spot-on.
Keep posting here and asking questions. Taking the time to plan a little before deployment will save you hours of head and heartache