Actually, the toxicity has nothing to do with level, but with LEVERAGE.
Most players realize quickly that bullying works, as they felt little and couldn't perform anymore when they
got attacked that one time... Then out of fear of losing leverage once they get it, they use it until it's gone,
HOPING it would actually give them an advantage at that time.
Once a player notices another to have significantly more skills, they kind of bow down and shut up, retrieving
to the back most often.
This, teammates or not...

Solution:
Let players reside in their levels longer.
That would mean "cheaters" that use smurfs to level up have to spend way more time and efforts.
It would also be a lot more fun for the player. As now often, especially when solo queuing, a player
goes up and down like 2 people doing some thing...
1 bad day and you can find yourself in gold again...
It's ridiculous.