WIPocket

Bottom left. Matrix client.

mautrix/telegram is a bridge between Matrix and Telegram. It mostly lets users of Matrix contact their friends who use Telegram. It is not a fork of Telegram and has nothing to do with the Telegram interface. (Note: OP wanted to use the Telegram client with a non-Telegram server. If you know of a Matrix client which looks and feels like the Telegram client, thats what theyre after.)

How is this relevant?

Likely yes. See the termux-notification-remove command from the termux-api package. (You will need the Termux:API plugin.)

Run a ssh server on the phone and rsync stuff over.

I have a built-in “PDF Viewer” app in my GrapheneOS. (app.grapheneos.pdfviewer)

Why would banking be an issue? I get that its a target, but I really would expect a bank to take care of their TLS.

Well, was it locked?

How is that an exception? Sure, it is sandboxed, but I really do not consider that “degoogled”.