any advice for this mongoose? It’s not really a bmx, but if I were to buy it I’d just cruise on it, I really like the fat tire look

Even if it wasn’t a neglected rustpile that needs a new chain, rear mech, shifter, and brakes (at the very least) I wouldn’t recommend it. That thing is a kid’s bike, it’s going to be waaay too small for you unless you’re the same height as an average 10 year old. Even if you put some BMX bars and a looong seat post on it it looks like it’d still be cramped as hell.