OK. I found rclone (AKA RCX) from fdroid and installed it on my phone. Config was simple and it seems amazingly fast to access my NAS. When I say the config was simple though I should qualify that by saying I already knew how to connect WebDAV to my NAS. A new user might not, so the URL may be a problem. In the case of a Synology NAS for strictly local access it would be . . .

http://192.168.1.xxx:5005/SharedVolumeName

I'll look for that "roundsync" you mentioned and see how that goes. I can't see either being of much use in the Joplin environment as it already does WebDAV but I've got lots of other stuff that could benefit.

Oops. Couldn't find roundsync so I re-read your post and rclone IS roundsync. Sync doesn't seem to work on my phone, complaining it has no Wi-Fi connection (I turned off mobile data so Wi-Fi is the only connection) and since Joplin really has no use for that sync I'm not going to spend a lot of time with it. It's a great remote file manager though.