I’ll have to look at NextCloud. It does look polished.
Syncthing doesn’t have iOS support, except through Mobius, which hasn’t had an update for 9 months.
As stated above, I’m using Syncthing, and also using it to copy files around from Box’s folder ~/Library/CloudStorage/Box-Box/org on my Macs to ~/org on my Manjaro box, but not loving it. It works, but seems fragile.
I’d also just like to put folders where I want them, but that’s a Box, Dropbox, iCloud, OneDrive, GoogleDrive, et al. thing.