The explanation is quite convoluted. Could you upload screenshots of the Syncthing Web GUI from all involved devices? Please make sure to unfold the folder info, so that all paths and shared devices are visible. The Web GUI can be accessed on Android through the left slide-out menu in the app.
At the moment, it sounds like you’re trying to sync two Send Only folders into a single folder on a remote device. While this will still upload the files, you will always end up with a permanent “out of sync” state on the sending side.
When it comes to deleting already synced files, etc., the simples solution is to just move them on the receiving side to a different folder. Alternatively, you could also utilise file versioning (see https://docs.syncthing.net/users/versioning.html) for this purpose, also on the receiving side.
There’s no option to ignore files by date, etc., but specifically with videos and pictures, which normally have dates in their filenames, you can utilise character ranges to target only specific time periods (see https://forum.syncthing.net/t/sync-files-from-a-specific-date-onwards/17651).