Did you have any troubles with this partitions encryption (luks)? Or you didn’t try?
I have not tried with encryption but I cannot see any reason it would not work. Encryption occurs at the block device level so I would think it should be fine.
Long-term is how long? Several years? Can you recommend something better?
Magnetic media (e.g. “hard” drives) is still the best solution for long term storage without requiring a power source.
Any NAND flash device (SSD, NVMe, USB drive, etc.) is fine for interim storage of data and can be used long term as long as it is either constantly or, at least periodically, connected to power. Over time, without power, they will lose integrity and there is the chance for data loss. However, even with power, over long periods of time (most research indicates ten years or so), they can still degrade and replacement should be considered at some point after to avoid data loss.