curious if you have any opinions on openVPN, since it's not in your alternatives. opening remote access of any type is very far down my priority list, so VPNs are one of the least defined "here be dragons" areas in my own self-hosting plans.
syncthing is just wonderful, though, can't recommend it enough. it really is one of those magic foss applications that just magically solves a problem that would otherwise be a thoroughly inflexible service with mass data harvesting or a subscription fee (or secret third thing: both).
the initial appeal of syncthing for me was being an actual solution to my very lazy setup of a desktop hosting common files as an NFS share that other devices can edit. this is the sort of thing that works perfectly for one weekend and then immediately creates woeful copying/updating debt any time a laptop leaves its home network, which you may recognize as a thing laptops are explicitly built to do. i am very smart
i'm using a very simple system of one main device that keeps version history, and anything else just syncs with no extra data on its end. this can really scale up as large as any reasonable home user would want, though. it's funny that you mention rsync and borg, because if you poke around in syncthing spaces you will absolutely see people saying "syncthing is not for backups, please create an actual backup plan, we are begging you"
lawrence systems on youtube has several videos on deploying or configuring syncthing in various use cases, as well as comparing it to nextcloud. disclaimer: i don't know anything about him beyond being a syncthing booster. there's thirty more minutes of level1techs every 48 hours, i can only keep up with so much
the main problem i've had with syncthing is that my most frequently used files can end up out of sync, although this one is really on me for forgetting to save changes before i walk away from something i'm actively editing. syncthing's solution to this is to fork the mismatched version into a sync-conflict file, so nothing's getting deleted. if you have a good way to do file diff this is just an occasional inconvenience.
a couple of other things i'd mention:
the stfolder dotfile that syncthing puts in the root of a share is not optional. if it's deleted, that breaks sync completely. you can just make another one and sync will resume like nothing happened, though. or so i hear. from other people. i don't delete things just to tidy up directories. hah hah
you can turn off global discovery to abstain from external relays, using your local network only. however, if you turn off local discovery, and have not assigned static IPs to synced devices, then none of them will know where the other is and sync will break. or so i hear. from oth
