Hey Mari!
Yes! You’re right on the money with your guesses :)
So for the first one yes, the person was really into something but no longer keeps up with it. So if a person “fell off the bandwagon” for a specific show or fandom, that means they used to be big fans of the show who would closely follow what was happening, but decided to stop or just no longer kept up with that show or fandom.
For the second you guessed perfectly.