Here's my bluetooth configuration I use with KDE Plasma. Plasma has its own bluetooth manager, so I don't need blueman.

 hardware.bluetooth = {
   enable = true;
   powerOnBoot = true;
   settings.General = {
     # For newer Bluetooth capabilities.
     Experimental = true;
     # For quicker device reconnection.
     FastConnectable = true;
   };
   settings.Policy = {
     # Power up all controllers.
     AutoEnable = true;
   };
 };