I think trading holmes, unless you have a stunning offer is a bad move. He has a 5M/yr contract, probably could be kept for 5-8M/year. You will not find that kind of production at that position for that cheap anywhere. Plus, he actually likes it in Sacramento.
As for moving forward, i would try to put center a large trade around bagley, buddy and Beli/parker and a first (maybe 2) for a major star like KAT. I'd be ok with moving some combo of the above for a player like Jerami Grant or Jaylen Brown as well.
Fox
Hali
Barnes
KAT
Holmes
Or
Fox
Hali
Brown/Grant
Barnes
Holmes
Fox, Hali, Barnes (probably moved to 4 in a grant or brown senario) should be kept (again unless you get a major offer for barnes). Barnes is probably is 3-5M/year overpaid, but the tangibles and intangibles (including community work, leadership and being an all around glue guy on the court and locker room) is worth it. Afterall, he is one of the very few who can get his own shot (often whenever you need him to) and is always the lead defender (better defender than the numbers show consider the dreck of defensive players he is covering for). I also think you could try to move bagley, bjeli and 2nd rounders for Markkanen and filler.
Fox
Buddy
Barnes
Markkanen
Holmes
I'd stay as far from Porzingis as possible. He has reached his ceiling and isnt making much headway towards a championship on a team with a top 5 player in the league. He isnt going to propel the kings into the playoffs or contention (nor would Markkanen but he is much cheaper).