So I have a group of about 8 friends who despite enjoying DnD aren't able to commit to running a whole campaign. To solve this issue I decided that all of their characters will form an Adventurer's Guild/Mercenary Group and whenever 4 or 5 of them have time to play I can run a one-shot for them. However I'm not sure how to handle leveling in a campaign like that.

So far I've come up with three ideas for how I could do:

-I treat it like a normal campaign where as they complete adventures everyone (regardless of who was actually in the session) gains XP, newly rolled characters come in at the same level, and then they eventually reach level 20 I give them one last epic adventure and restart. (This is the easiest character wise but means if my players want to play a more basic adventure then they all have to make new characters)

-I treat it kind of like an MMO, only the adventurers that go on an adventure gain XP, whenever a player decides to roll a new character they come in at a set level and have to work their way up the ranks of the guild. (This is probably the most realistic approach, but would be really punishing to the less active players)

-I have each player make a Character sheet for each level 1-20 and they just use whichever one is appropriate for the specific adventure. (This is the best way to make a wide range of adventures but it also removes the feeling of character progression over time).

Has anyone done something similar to this, and if so how did they handle leveling?