I use resilio sync across my work and personal phones and I use it to backup my phones to my nas. No need to turn it on/off, it's accessible only via secret key or shared link and it's encrypted.
I know sync isn't foss but it's the best one I've found over the years and it just works.
Love resilio. Fantastic for travelling to quickly pull up your passport or visa or train tickets you reserved before. Just leave a computer running at home (like a home server).
Mobius Sync is an iOS app for it. Free version has max directory size 25mb(?) but dev seems to have good attitudes; it's something I wouldn't mind paying for.
It's a bit more consumer focused, easier for Grandma kind of thing. Syncthing isn't hard, but does have in my opinion a bit more difficult of an initial setup with features like introducers that can make things difficult if configured wrong.