Techniques for leveling small section of subfloor
This looks like a job for self leveling compound. I would probably do it in 2 pours it looks a bit thick for a single pour. What kind of flooring are you installing? It looks like there will be enough height at the door for just about any kind of flooring Once leveled.
Folks that haven’t worked on old homes don’t realize true dimensional lumber was once used. Plywood is thinner the 2x4” are thinner today and may be the cause of the height issue.