A couple of addenda:
1. It does seem like Synology actively uses eCryptFS for encrypted folders, so maybe it is not as abandoned as it looked.
2. I also saw discussion of EncFS, but there also seemed to be indications that that was abandoned.
3. I I'm viewing this as separate from the issue of encrypting the entire partition with all my files, because the point is to have these files encrypted with a separate passphrase. Since they seldom need to be accessed, this will hopefully add a bit more confidentiality. For the same reason, performance isn't much of a concern.
4. I realize that the data may still sometimes be present in swap, but again if the assumption is that access is infrequent this is at least only rarely the case.
5. I know that for specific file types the files or certain applications there may be a mechanism for password protection or similar measures, but I figured it made sense to just have a generic solution for arbitrary files.