Desk pushed against the wall with monitors on it, how much is the desk itself wrecking my low mids

Measure before you buy anything, because this room will surprise you and everyone's guesses including mine are worse than one measurement. A calibrated measurement microphone and free measurement software will show you the actual dip, its frequency and how much moving the desk 30cm changes it, and the whole exercise costs an evening plus the price of the mic. I moved my desk in 10cm steps taking a sweep at each one and found a position that was substantially flatter than either extreme, which I would never have found by ear. Do that first and you will spend money on the right problem instead of the fashionable one.

Both suspects are real and they are the same kind of problem, which is a delayed copy of the sound arriving and cancelling part of it. The desk sends a reflection up off the surface into your ears a fraction after the direct sound, and the wall behind the speakers sends one back out past them, and both produce dips rather than boosts, which is exactly why things go thin rather than boomy. Order I would try, all free: pull the desk out from the wall as far as you can bear, get the tweeters at ear height on proper stands or at least on something rigid, angle them so they meet just behind your head, and put something absorbent on the desk surface between you and the speakers when you are critically listening. Then reassess before spending a penny.

I moved my desk about 40cm off the wall, feeling ridiculous the entire time because the room is small and it now looks stupid. The low mids improved immediately and audibly, and the first mix I did afterwards translated to the car without any of the usual corrections. Zero pounds spent. Try it before you buy foam.

Disagreeing on the priority order. Symmetry comes first: if your left speaker is 40cm from a wall and your right is 2m from one, nothing else you do will fix the imbalance, and no amount of pulling the desk forward addresses it. Get the listening position symmetrical in the room, then worry about the desk and the front wall.

Stands rather than pads if you have the floor space, and it is not really about decoupling. Pads stop some vibration going into the desk, which is worth something, but the actual win from stands is that the speaker is no longer sitting on a large flat reflective surface at all and you can place it independently of where your monitor and keyboard need to be. Decoupling is the small effect that gets marketed, geometry is the large effect that does not.