This might depend on where you’re uploading/how you’re playing this file, but you could add a thumbnail to the audio file? I know that vlc and mpv will play your audio file and show the thumbnail, but I’m not sure if YouTube would take that. Not sure if this is exactly what you’re looking for but it is pretty efficient.
Yes, very. I’ve seen people do a similar thing with a separate encrypted home partition which is decrypted by a key stored in your encrypted root. However, I’d strongly recommend you use an LVM on LUKS setup (this is what I do). That way you decrypt one partition and you don’t have to mess around with keyfiles. https://wiki.archlinux.org/title/Dm-crypt/Encrypting_an_entire_system#LVM_on_LUKS