Possible, and very poorly thought out, solution to the promo problem.
Since promo games cause such a big issue hows about they work like this:
A player reaches the 100LP like normal and goes into their promo matches. They must play all three/five matches and if they win a majority of the matches they advance to the next rank. However, if they lose all of their matches but that players MMR is still higher than the division they're in currently they can still advance.
Stops a bit of the frustration around games when you easily get into your series multiple times but just cant get over that last hump and avoids players just throwing their matches to advance without playing because they might not rank up with three losses.