Then there's the (to me, still open) question of how to best use the actual tape storage capactiy... Since my hardware is newer than LTO-5, LTFS (https://github.com/LinearTapeFileSystem/ltfs) is an option for convenient access, especially listing tape contents, but that could make it hard for other people down the line to restore data from the tapes I create.
It's probably safest to assume that tar will always be there, at least wherever there's tape, too. GNU tar also handles multi-volume/-tape archives, which seems like a necessity if you need to back up amounts of data that exceed a single tape's capacity. Then again, if you want to use encryption with actual tar (important for the kind of data I need to archive), your only option seems to be piping the whole archive through something to compress the stream, which will make accessing individual records in the archive opaque to the drive itself... and you can't just dispose of individual keys to make select parts of the archived data go away for good, either.
Also, I would like to conserve as much tape as (conveniently) possible in my archiving adventure. There's "projects" (i.e., top-level directories of directory trees) that consume more than one tape of their own, and then there's smaller projects that you can bin-pack together onto tapes that can fit more than one such project.
I've started implementing a small python wrapper around GNU tar to solve a number of these problems by bin-packing projects into "tape slots" and also keeping track of tape-to-file mappings in a small sqlite database, but a workable solution for the encryption problem(s) is not something I managed to come up with yet... If someone has an idea (or better yet, a complete and free implementation of what I am trying to hack together :)), please be so kind and let me know!