It’s true I want hours of playback, but in the car, it’s possible to draw up to 100 watt from the outlet and in trains you have 230 volt outlets. At least in Denmark.
On a plane, you usually only have USB, but I’m not sure I like the idea of setting up WiFi on a plane 😅
Your suggestion is “good enough” and what I do now, but as you say, it could be fun to make a little project out of it. Figuring out what works and what doesn’t.
It’s not a bad solution and I have used eSIMs before when I was traveling in Asia. However, all of these “unlimited” eSIM plans has a lot of buts. Either the speed is limited to close to unusable for streaming and/or you are limited to only use x amount of GB when using hotspot.
Also depending on the country, the coverage can be awful.
If you can get 5 people for the Apple One Family plan, it's worth it.