Is there a maximum level of enemy progression?

If I were to play through a game to the point at which I am ready to start the final mission, but continue to play normal UFO assault missions instead of starting the final mission, what is the maximum level that the alien progression would stop at?

There must be a maximum level, because there seems to be a maximum number of aliens for a given size of UFO, and once all but the commander are sectopods, it probably can't go further - but would it ever get this far?

Solution 1:

Progression will stop. You will have UFOs commanded by Ethereals, with Muton Elites, Berserkers, Heavy Floaters and sometimes a cyberdisc, one time I even got a Sectopod in a downed UFO mission, and a pinch of Sectoid Commanders. But from here there is nowhere to advance.

As you said yourself, there's a limit to the number of aliens per UFO, and there is a limited number of alien types and weapons, so eventually, they will stop advancing. However, as time progresses, you will get more larger UFOs and less smaller UFOs.