Agree- it is a logjam at CM right now. If we are playing 3-4-1-2 or the close variants we only will need 4 CM next season. As you say, now that we have Gallagher on a long term contract and high wages, he will have to be played.
If there was ever a time to go for pure youth it'd be this next season where we have a free hit a rebuilding year. Galla, Bergvall, Sarr and Gray would be able to hold down those positions, and Gray with his flexibility could easily get some extra games filling in at RWB and RCB too. Of course the sale of Bentancur would be complicated with his recent extension, but you'd hope the club would find a way of getting it done.
If we sell Romero it may be advisable to retain Palhinha also to play RCB and occasionally at CM for us. It's an interesting option, and he has looked good for us there. Palhinha and VdV flanking Vuskovic sounds like the makings of a pretty good back three imo.
If we're going to carry 5 or more CMs though it'd be best to loan one of them out to ensure they get enough games to develop. Seeing the bone-headed reluctance of our org to start Bergvall in his natural 8 position, I'd hope they send him out somewhere where can simply play and learn his trade.