So currently your MMR system is a solid system no doubt, after reading how it works i can say that its not the system it self that is the problem, the problem with such a solid system is that it requires a decent playerbase to be able to match players correctly, the game does not have that currently.
My idea is to remove the MMR system entire for the moment, until the playerbase reaches an average per day that can sustain such a heavy system,
Now let me explain what you could replace it with for the time being,
Ideally imo a difficulty selector at the side of the adventure screen would be a better choice, you could level lock the difficulties. so for example
10%, 30%, 50%, 70%, 100%.
then only new players will able to pick the difficulties that would suit their level, also to add to this, make it so if you select a high enough difficulty but it cannot find players of said difficulty, the game would ask you if you mind lowering your difficulty. i mean overall, i'd rather faster games than unfair games.
the thing with this style of suggestion is no-one would ever play at a difficulty above their player level, New players would be still able to play at a completely acceptable level to their skill level. If i high level player was with them, yes it would be a breeze for said player, but thats far far superior to having 2 players quit right off the start, then the other just dying every 10 seconds, while you try to solo a mission intended for 4 players,
the worst part of these matches though, is the MMR usually drops like 10% from the high level players MMR, yet its still too difficult for the lower levels, thus usually ending up in a loss, This is big kick in the balls fora high MMR player, why? because the difficulty is much lower than their current MMR, when you lose, you lose like 5-7% MMR.
This takes about 5-10 games to re-increase to the same level and is too punishing for the person that tried their upmost hardest to complete the mission.
The only major downside i see to system, is that if your the hardcore mentality sort of player, where any challenge is an acceptable challenge, you are now level locked to what you can do. Though that said this sort of player, like myself would find ways to speed up said missions, find ways to make them go flawlessly, this is enough to keep that sort of player interested till they hit the next difficulty tier so to speak.
Anyways its just a suggestion, i hope it brings new ideas to the table for your development team. Thanks for reading