Now moved to a more powerful machine, but the concept is still the same.
This setup allowed me to conveniently stop using google drive.
I used to run Syncthing on an old mac mini with two 2.5" HDD running FreeNAS (previous name of TrueNAS).
Haven't tried it while out-and-about, though that's not really my use case.
*RPi4 NAS has one external thumb drive for flash storage and two spinning rust drives, one for storage, one for backup; all connected via USB. Occasionally I have to plug-unplug the spinning rust drives if the system reboots, as the drives go to sleep.
I finally got rid of it because of the awful mobile experience, but frankly I shouldn't have been counting on that anyway.
What problems were you experiencing on mobile?