Convertible Car Seats

My son is 4 months old but 21 pounds and 27 inches long. He’s just getting a little uncomfortable in his infant seat. We just bought and installed the maxi cosi Magellan max and I’m not a fan. It’s very comfortable for my son but I hate the harness (it’s awkward and I can’t get it tit enough on his chest but cuts into his legs) also it doesn’t seem secure in the car. No matter what we do it wiggles pretty significantly.

Any suggestions? Im looking for safety obviously but I like comfort for the babe and recline is nice too.