What is the 5v 3 pin header used for?
I think most AM4 motherboard fit the criteria for USBports on the rear panel and it becomes especially easy if you have a case with wireing for USB2 and USB3 front panel ports. But since you've a 2600 you'd want to pick a B450 board as B550 isn't officially compatible with Zen+ CPU's; avoid B350 as they have largely inadequate VRM's. The X470 (and a few X370) would be OK but only if you have a need for lots of add-in PCIe devices.
In ATX size, B450 Tomahawk Max is good choice or Asus TUF B450 Pro and Strix B450-E, in mATX, B450m Mortar Max and Asus TUF B450m Pro-S.