It sounds like you have had the influx of new players, and you have improved.
The problem is of course, you are now playing against people who are better than before especially if the matchmaker is working, so your win rate of 59% wasn’t going to keep going.
The game put you into harder and harder games until you hit 50%, but also, your MMR and SR will keep going up until you reach that point.
Welcome to higher ranks, sadly. In lower ranks people know they are not amazing, so they tend to be more “chill”
As you get close to plat, people start taking the game more seriously, and they tend to be more toxic.
Once you get into Diamond, the toxicity starts going down again (at least it was for me) - because actually remaining chill is a skill which helps people win games.
I REALLY wish they could fix this, but it is hard to make your community chill. It CAN be done though. (See FFXIV for an example of people doing exactly this - it is chill even for MMORPGs).
I wish I could give better advice there, but the problem isn’t you, and honestly, while the matchmaker isn’t great, it isn’t the matchmaker trying to put you in with bad people.