as an example, I do this currently and those are my ignores:
- Make sure, you do NOT sync any databases or log files which are in permanent change by apps. That’s why I’ve excluded WhatsApp’s DB folders. They may raise conflicts, pull errors, battery drain because of errors and so on.
- Make sure, you do NOT sync the /Android folder. I’ve tried syncing it some years ago and ended up putting new sub-folder ignores every week which wasn’t easy to maintain. From my experience: Very little kind of apps keep their data on /storage/emulated/Android/0 in a use-able form. For me, it was OSMAnd, for example, keeping map material downloaded from OSMAnd servers.
- Like Audrius said: On the one hand you can get a pretty good backup of your USER data which means FILES (e.g. photos, videos, music, recordings, manually to-file-exported app settings, etc.), but due to Android restrictions you can NOT get a use-able backup of your app settings, internal files an app has downloaded on /data/ partition (…). I’ve tried this in my root days and had just a large bunch of useless files on my NAS hogging up my phones sync performance on the other side without any benefit when I had to recover my phone.
Long story short: Try to ONLY sync your USER files from /storage/emulated/0 without special folders (/Android), logs (/Tasker/logs/*), databases (/WhatsApp).
I hope it helps you a bit.
And yes, Android has in my eyes proven not to be a long-term “stable” platform for Syncthing as OS changes often offend the app to even exist (see Android 10 scoped storage, for example, which will finally hit Syncthing in Android 11 according to Google’s nowadays announcements). That’s why your NAS is recommended to have a backup mechanism in place to BACKUP your synced data which came from the phone. Android glitches (coming with an Android upgrade) could cause Syncthing to fail - Syncthing cannot be held responsible for this and is a SYNC solution, so a good BACKUP on the nas side, e.g. copying snapshots of it to an external hard drive every night like I do, is advised.