It’s pretty simple, if your rating is below your internal rating you’ll get 300 rating per win, if it is above your internal rating it will be 100-130ish. Your internal rating is not how you finished before the patch.
This is the same system as they use for Overwatch and it is completely awful. The best thing you can do is start over with a new account or grind it out.
I thought I would try it out again after quitting to see where I was at and it was fun getting 300 for every 1st thru 4th win, but once I reached 2000 rating, my rating per win is 104 and 4th is 19. So even if you improve, your internal rating will keep you down and you’ll have to grind really hard to get wins even though you’re better than the system thinks.
It’s whatever.
Same ole Blizzard; Different game.