Under desk laptop mount $10
Wasn't certain if this could hold extra keyboards and mixers under desk, but meant for laptops when attached to monitors and peripherals above desk.
USB Hub RGB Monitor Stand $10
I feel like you can get this to help you with extra USB ports, provide extra mood lighting. I believe there's a white version of this at Five Below.
Cable Management undermount tray $25 : could be used for organising and hiding extension cords, hold headsets not in use, etc.

Are you able to change out the desk legs, is it a flat top, foldable, or those Home And Garden sets that will absolutely not allow any modifications?

White Table top IKEA $50 : They have table tops that you can intermix with different types of legs and it's easy to screw in on the underside some cable management, shelving, and keyboard options. They are able to hold the PC tower and two 30 in monitors with ample space to hold all the randoms.
White table leg $7.50 per leg: Even comes in an option of black. They successfully hold up to 90 lbs worth of peripherals and gear for 5 years now. Probably could go more if the table top was solid wood.
Adjustable desk legs $190: If you want to upgrade any upgradable desks......