Help with choosing a class for upcoming campaign

Hi all,

Our group is converting to pathfinder 2e, coming from D&D 5e for another homebrew campaign.
I've been having some doubts as to what class to pick that'd fit our party, as I'm not quite familiar with the system just yet.

Currently it seems like we'll have a Bard (Occult spell list), Witch, (Occult spell list), Monk (Drunken Master) and Gunslinger (Vanguard).

In 5e I enjoyed playing casters a bit more than just martials. Currently I've set my sights on either a Magus or Summoner (Primal spell list), but I'm not sure what'd be the better pick for rounding out the party and what build would match best alongside it. I'd love some input on what you think would round out the group the best. Or if another class would be a better fit, I'd love to hear it.

Thanks for reading & for your input!