PC side:
I use an app called SyncTrayzor https://github.com/canton7/SyncTrayzor that works well on the pc. I just add the folders I want to sync. Back up ALL the files you want to sync because you can definitely lose your data. You should start on the PC, or android, where the source file currently reside. In SyncTrayzor you will be using the Folder ID to sync ;the data to your other devices. Once you sign into syncthing you should see the folders available to share on the other devices. You will need to designate the local folder where you want the synced files to reside. You can sync multiple folders between different devices.
Android side:
This is where I had most of my problems. Finding the file path is a PITA! File browsers show different names for the same location. On my MOTO G6 the location I wound up using was “/storage/emulated/0/sync”. As I understand it, this is a location on the SD card that is at an mirrored locating in the system.
Plan the locations out ahead of time. If you start deleting and reinstalling Syncthing you probably will lose your data at some point. I would make sure you have a few backups in case you find something missing at some point.
As a side note, I use KeePass on my pc’s and keepass2android on my phone. I have found that if you use the QuickUnlock feature on the Android the encrypted database won’t update and you can get out of sync. If the database has been changed in multiple places it can be a mess.
I have not used SyncThing encryption since the only personal information I keep on my devices is the encrypted keepass database. I had tried the encryption at one point but wound up with multiple keys that can’t be deleted and it just got too confusing.
I’v had this running pretty stable on 2 pc’s and 2 Android phones for 2 to 3 months. I like the peer-to-peer aspect. It would be really nice to see syncthing incorporated into, or directly supported in Joplin. Seems like a natural, both being open source.