The Hornets will not be able to sign a FA center if they resign Monk.  Well unless they get someone really crappy and cheap.  Like a downgrade from Biz/Zeller.
Monk's cap hold is roughly 16 mil.  The team projects to have somewhere north of 23 mil in space if they renounce Monk.  If they keep Monk they can't sign anyone of consequence while his 16 mil QO is on the books, and if they sign him for 12 mil then they have maybe 11+ for a free agent center.  That's not going to be enough to pry Holmes away from the Kings (they can match 11, but not anything more than that).
Edit: I should also add that John Hollinger's BOARDS posts where he tries to project what free agents are really worth ... the one where Dovonte' was put at $20 mil ... lists Monk as being worth less than $5 mil per year.  Hollinger also lists Monk's QO as far less than the other site where I looked, at $7.3 mil.  If that's accurate then maybe it's theoretically possible to sign a FA center, but still pretty iffy given Charlotte's limited cap.  Devonte's lower QU makes it easier to get a center.  Just QO him, sign a center, and then resign Devote' going over the cap.  Link is paywalled.
https://theathletic.com/2713836/2021/07/20/2021-nba-free-agency-shooting-guard-rankings-by-bord-no-stars-but-several-starters/