Maybe it can be hacked together with Syncthing: have your phone's camera sync with an inbox folder on the desktop, have the desktop pick up the files and transcode them with handbrake, then move the original out of the inbox. This will cause Syncthing to sync the deletion back to your phone, and sync the transcoded version back on your phone.

I'd also check if you can just change the bitrate in your camera app's settings in case there's a way to lower the quality there. Could be noticeable, could be just as good as handbrake, never know with hardware encoding.

I think you could get something working using termux to install and run ffmpeg on your phone. Then using an automation app, e.g. if this then that, you could run a script in termux to re-encode the files. And automate it at night while charging.

iPhone here.

My Synology NAS can do this somewhat. The local videos/photos gets sync’d to the NAS. It then becomes available in its own app, when streamed its transcoded. Over time the local photos/videos are removed.

FFMPEG Media Encoder can do the main part. You will need something else to handle scheduling though.