why am i getting matched with 5000+ kill diamond rank players every match?

Well, for starters most of those guys probably aren't playing with randoms, they are playing with their friends of equal skill using Microphones often.

So that's probably the number 1 reason you get matched with bad players, because the crazy players tend to have played enough to have met other good players that they team up with instead of playing with randoms. 

I'm a "casual" diamond player myself. 

For me it's the 40 000+ kills players that smoke me lol 😂

Also, Duos I think is filled with tryhards right now probably. 
It was when it was first released anyway. 

Casuals are playing normal unranked. Tryhards are either playing duos (because they only have one other friend lol)

or they are playing diamond/pred ranked right. 

So yeah... that solves the mystery I think no?

Play normal dubs, invite good players when you meet them, teamup, get good team synergy etc. problem solved. 
Game is still going to mix you in with the psychos tho. 
No way around it. 
But at least that always makes it worth trying to improve and learn and that's part of the... fun... right?