I think the way I'm going to start this PF2 game is to just drop the party off at a starting mission location outside or under the city. Just something simple like collecting reagents for an alchemist or something, as part the adventuring guild. Maybe make adventurers register there. If any person gets X level of martial/magical power they need to register unless they have specific exemptions.
It feels conscription-y, but not sure if that's the tone I want. I think I won't go that route.
Edit: Also considered Troupe play a bit, but I don't know if I want the players to be splitting their attention? Then again, fuck it, back up characters are nice usually.