Sounds typical, my Oneplus 7T is on 81% after just over 3 years. I wouldn't get hung on it too much, if the battery still does what you need, why worry! Oneplus did used to offer very, very cheap battery replacements as well (21 quid ish on mine iirc).
I wouldn't worry about it if it does what you need.
Not sure how long mine would last without charge, as it's usually on a charger on my desk. That said, it's an iPhone SE, about 5 years old, and says it has 90% capacity. I don't use for much beyond calls and texts though.
