As an EU player I can confirm matchmaking is absolutely terrible here aswell. Like the OP I'm forced to always pick my best champion if I want to potentially even have a chance to win. And I'm speaking from a perspective of a casual player I'm constantly matched against GMs, Masters or onetricks that sweep the entire team just because I've been playing a lot of Maeve and learned to do reasonably well so my hidden casual elo skyrocketed to the point where I can't have fun anymore. I either get matched with 4 new players who refuse to speak english and complain that I can't kill the enemy sniper while he's getting corvus + jenos pockets or I get matched with some seasoned GM players who stomp the games and I don't even have an opportunity to do anything besides watching how I get the dull "Victory" screen and moving on. Since hirez is so die hard in favor of fast matchmaking instead of balanced one i'd suggest a compromise:
Two options of matchmaking "fair" and "fast"
"fair" matchmaking would have a warning once turned on that it may make the queues extremely long but actually be fair. Aka don't match people who just installed the game with seasoned grandmasters or vice versa.
"fast" matchmaking would stay as is. You get fast matches in exchange for your sanity and possibly a stream of losses.