@fuzzykeuh
I am also a Diamond player on my main (currently D2, about 2.7 overall k/d, 3k dmg badges on several legend etc) and I honestly think the SBMM in casual is quite fair when you are playing solo. Sure, it feels like I am more likely to get matched with worse randoms than better but that can be explained by the average skill distribution of the player base (last season, like 70-80% players were gold or worse).
However, it sometimes feels like the average skill level of the competition is upped significantly whenever I play with a full 3-man premade squad. Perhaps the casual matchmaker is designed to prioritize full premade squads vs each other and I guess statistically, most skilled players play premade and most noobs play solo so that would explain this.
PS. Are you really serious when you have 4-5 predators squads, i.e. up to 25% predators? My matches, also solo matches, usually have more like 4-5 predator players in total, and since Im Diamond, its a totally fair, even kinda casual, match up.