For 1. you don’t necessarily need a VPN. A reverse tunnel using SSH should be sufficient and is easier.

Text-based subtitles mostly work. Picture-based subtitles require transcoding or a client that supports them.

I have a RockPro64 and 2TB SSD connected with USB. I measure it at around 5,5W so in a month thats ~4 kWh electricity. For storage I think its not feasible to host it on a cloud, see https://jellyfin.org/docs/general/administration/storage/

yt-dlp -x -f 251 https://music.youtube.com/watch?v=9CWTig2kBKE

I upload all my music to a selfhosted Jellyfin server. Finamp is a great Android/iOS app for Jellyfin that has offline play.