It'll definitely be better but can't tell how much. Using it daily for 2+years i haven't really noticed any degradation tbh. If you plan on keeping the phone for another while you will have to replace it eventually sometime this year so why not do it now.

As long as it's over 80% it's fine. Got an 11 pro max at home with 81% and it holds up pretty good, below 80% it gets throttled so will need a replacement then

What's your battery health? Seems a bit early for an iPhone 12 battery replacement