I don’t like being that guy but its probably not the leavers.
I was lower into bronze than you are, at some point I even went under 500SR and I climbed up to high gold on a trash laptop with under 40 fps and over 100 ping and myself sometimes being the leaver because I would just get kicked out of the game (which btw already loses you 50SR, more than just losing the game) because I live a long way off from any server.
You can climb even in terrible situations, keep it up and give it your best, honestly is the best advice I can give you.