Since none of these require a Raspberry Pi to run, I would suggest using a mini PC (with an Intel N100 or similar) instead of a Pi 5. With all the accessories needed for the Pi, a mini PC can actually be cheaper and of course a lot more powerful. Since the Pi 5 is very power-inefficient, a mini PC can even be better in that regard too if that matters to you.

Especially for Jellyfin a PC with an Intel CPU with integrated GPU is awesome, since Jellyfin supports hardware transcoding with that.

I was thinking about doing something similar and was considering running Android on a Raspberry Pi. There are unofficial LineageOS builds for the Raspberry Pi. I haven’t tried that yet, but I guess it should be possible to use the Jellyfin Android app on such a setup.

Getting certs from Let’s Encrypt should work fine with any provider, even if you can’t open any ports, since they do support DNS challenge.

Mine runs a little under 18 W with one 8 port managed switch, a DSL modem, CM4-based router, a tiny Wifi AP, and an Intel Celeron J4105 based mini PC server.

BirdNet-Pi is awesome. Highly recommended for anyone who likes birds. The BirdNet app for phones is also nice.
Btw, BirdNet-Pi also works fine on the non-plus Raspberry Pi 3.

Any backup software that supports incremental backup should work similarly bandwitdth-wise. I like Restic. You can even do incremental backups with plain rsync, if you want. If your data does not change much, than you should be okay. For the initial backup run it would be helpful if you have physical access to the remote location so you can bring a full backup there without having to upload it through your slow uplink.