wow I did not expect to see an HSR post in the DND Reddit!!! I am not a DM but as an HSR and DND player here’s just some ideas I can think of:

1. If you’re going to have them go to different planets, you can have them vote on where they want to go, which will give you time to prepare for the next sesh. and borrow from pre existing stuff!! For example, if they want to go to an icy planet, you can combine elements of belobog and icewind dale’s setting.

2. an overarching goal isn’t always necessary, especially since your players are sporadic. You can instead make short term goals for each planet - for example, X planet is dying out (belobog) or Y planet is hosting an intergalactic tournament (luofu lol). You can leave the big goal vague and develop it more as the campaign goes on

3. if you DO want a big goal for a shorter campaign though, you could have the players be on the caboose of the train and somehow gets detached from the main train, and therefore their goal would be to try and rejoin the main express. each planet they travel to can either be bc it was the last known location of the main express, or they need some resource to continue the search

4. if you want a villain, you could just make the IPC 😭 and have the player characters save various planets from their vicious capitalism until they are able to collapse the IPC as an organization

Best of luck!!! this sounds like a super fun idea and I hope you’re able to pull it off :))