Team game rating has some issues at the moment. One of them is about how to deal with solos vs premades.
Most of these issues are already discussed in this thread. You might wanna have a look at this thread. You might already refer to this thread by refering to mass inflation. The idea of dealing with solos vs premades is already discussed in that thread as well. You might find some good suggestions of dealing with this issue as well in that thread.
The mean issue with team game ratings is inflation, but the second largest issue is probably how to deal with solos vs premades.
This is only how quick play works. It is not done this way on ranked. MM for ranked really looks at ratings. But i agree with you: most games are pretty much unbalanced and winning them is a coinflip a the moment.
Not every TG player plays a lot of 1v1s. Some might even have no 1v1 Elo at all. Therefore this isnt really an option. Dont get me wrong: 1v1 ratings are much more reiable and therefore most likely a better indication of someones skills. Fully agree with that part.
If we got a good TG elo calculation, this isnt also needed. Then we just can use the TG rating. But the current issues with the TG rating needs to be fixed first.