You can build it in Godot and export it to your android. Alternatively, you could build a webpage and access it via your phone browser.

Godot is how I’d do it. I’ve done something similar before.

Try handbrake maybe https://handbrake.fr Might not be exactly what you want but something to look at while you wait for other answers.

If you are actively making 4k videos through, I think it might be time for you to upgrade.

Ora.pm can do what you want I think.

Can I ask why your trying to drop nextcloud? I only really hear people raving about it. Been considering setting it up for myself.

Maybe also check out anytype