I'll say, I had a nearly identical situation. 3-year starter with 2 natties (in years 1 and 2, lost in year 3) scrambling QB entering his senior season owning every school QB record. 98 speed, pretty great passing stats, but I had a RS Sophomore 5-star QB with Elite dev waiting in the wings. The senior starter was a 4-star with normal dev and was totally capped out. They were about even in overall with the Sophomore becoming a natural 81 to the Senior's 80 (with tactician boosts roughly another +5 to each overall), but the Sophomore was a little worse in the key stats (with room to grow) and slower with 82 speed.
I made the hard choice and went with the RS Sophomore. He developed into an actual 99 by his senior year with no slider manipulation. Best QB the school has ever seen in 3 years starting. Broke all-time single season and career passing TD records, all conference and school passing records. The extra games will be invaluable. Careful though, as he will likely try to go to the draft as soon as he is a 90+ at the end of a year meaning you'll likely need some luck to bring him back as a Senior if you start him as a true Freshman.
You could also redshirt him a year for some extra development and start him as a RS Freshman next season, but then he'd be draft eligible after his Sophomore year.