I never played much MP, just completed SP campaign, and went back to check. Found that my APEX rank dropped from 14k to 19k. I do have a few level 20s but I find the grind for the loot crates pretty tiring(about 3 gold matches for 1 premium crate which cost $100,000 credits or about 45 min to 1 hour of gameplay).
Pay to win(crazies who want top ranks with best gear and character) and buy them with real money is not my style. Unlocking more of an item increase the rank of that item or character. E.g Equalizer X is when you opened 10 random rare cards that unlocks Equalizer. I->II->III->IV, etc.
I'm not motivated to play new game+ or continue multiplayer. So I'll move on. Good luck.