But before writing my backup to the cartridges, I tried reading their contents, and found that they actually came from a major film studio, with backups of raw animated film content on them!
Still did you try to recover any material and wach it?
Home users will really want to think in terms of the "raw capacity" imo. This is normally half of the advertised capacity for the older standards (I believe the newer ones have stronger compression that squeezes a bit more). LTO-5 tapes are 1.5tb raw, for example.
Maybe you'll get a little bit out of it, but a lot of the things you'd want to back up (and especially the bulkier stuff that really eats space) are already compressed. Family photo library, audio/video storage? JPGs are compressed, H264/H265 or MP3/FLAC/etc are already compressed. System images? A lot of application files are already compressed. Home user scenarios are not outlook mailboxes and database backups like the "official" scenarios.
Everyone would be better off thinking in terms of the raw capacity. "Compressed capacity" is nothing but a marketing gimmick. Even in enterprise use cases the compression ratios will vary, and the drive's transparent compression is unlikely to offer the most savings. If your data is at all compressible you should compress the backup yourself before sending it to the drive.
And most enterprises don't really care if their monthly backup requires 10 or 15 tapes. And zipping it all up beforehand requires even more space on the primary storage which is even more expensive than a couple dozen tapes