@ftxcommando your suggestion do disallow big rating spreads is crazy. Imagine you manage to convince a friend to try this game only to then be denied the option of playing with them. Are faf promotional efforts that good at attracting new players that you're ready to sacrifice this basic human interaction just to have ratings converge a bit faster? How are you gonna justify banning smurf accounts after you make that the only way of playing alongside your beginner friend?
If a change to tmm is really warranted because of this problem, my vote is for configuring the matchmaker such that it tries to find a similarly structured opposing team, rather than what you suggest. If a 1500 + 1500 + 100 premade is in queue, it's ok to match them vs 1400 + 1400 + 300. TMM is unavailable half the time simply because not enough players are searching for a game, any further restrictions on team compositions would only make that worse, and for bigger games, your proposal constitutes a harsher restriction than mine. But there's a delicate balance between wait times and match quality that needs to be struck, and if the scales are tipped too far towards the latter, people stop queueing because of the wait times, compounding the issue, so I'm not convinced the problem justifies the cost.
As for custom lobbies, in addition to the argument above about friends playing together, there's the fact that for many noobs, astro lobbies are the only option. No matter how much we wished it wasn't the case, if a noob hosts any other map, their lobby won't fill unless the title is "all welcome". This is because other new players rarely get in the mood to try something else. When they do, the friction imposed on them should be minimal.
Custom games can be reasonably well balanced if 2 noobs join, especially if the map layout allows the host to put them against each other, and it's far preferable to hoping a noob is willing to try hosting, and they're in the mood to pick a map they never played before, AND 7 other noobs decide to join. Not gonna happen until the nr of players online is an order of magnitude higher.
Besides, since when is global rating such a treasure to cherish when It can be gamed so easily? Why put up such outrageous barriers to play just to protect this flawed system from a bit of variance, when it's been the general guiding principle of faf development for some time to steer players towards ladder/TMM and league standings anyway?