@imsodin The fork already has Android 11 compatibility, using sdk 30.0.1 build tools and targeting api 30 level. The main change was in the AppManifest.xml (added all files access permission and legacyStorage = true). Then the intro dialogs were extended to prompt for all files access on 11+. I recall looking at AppManifest and FirstStartActivity did solve it so the SyncthingNative has access to the files as before plus sd card support on 11+ only.
Ref.: https://github.com/Catfriend1/syncthing-android/commit/545a9ffb1dd419d662ebc280eb05bd9352a56a56