Frank didn't have this offseason to train.
Frank played in a system that was about ball movement rather than isolated aggressive drives.
Frank has never weight trained before.
Frank is still growing (Allan Houston posted a picture pointing out that Frank is now taller than him).
Frank's jumper was still developing even in the French league, his mechanics and improvement were what made scouts so certain that he'd eventually be a good shooter moreso than percentages.
All of this is to say; Frank's going to get better, the only question is how much better? Players in this league, especially Euro players, tend to improve quite a bit as a shooters over the course of their first few years. Many of them develop athletically as well with weight training and nutrition regiments (just last night they remarked about KOQ being in better shape this season and he's been about to sign his third contract). Ball handling tightens up and decision making improves with experience for a majority of guards as well. None of this should be all that doubtful, it's commonplace that these are areas players improve. The fact that Frank's still growing into his body, has never lifted weights and has to adjust his attitude from a Euro system to an American style is all reason to think he's far from a finished product to say the least.
Here's the other thing...the kid defends pick and roll ball handlers better than anyone else the league, that's not an opinion, when he's holding oppositions to less points per possession than anyone in the league in that scenario. What makes that so important is that the pick and roll is utilized so heavily in this day and age. Defensive metrics are all really kind to the kid, he checks all of the boxes on the eye test too. He deflects balls, plays the passing lanes, understands spacing, is vocal...he has every aspect of not just a defensive starter, but a good defensive starter. There's a clear path to being an impact player on the defensive end here and it was already his role in the French league. Again, his offense needs improvement but in areas that most young players do improve in. But his defense is already advanced and if it moves to the next level then he can be an impact player by just becoming average in certain aspects, that's not mountain to climb. I personally think he'll be better than average offensively with time and development but even a conservative projection on his offense improving would leave you with a pretty good player...not Donovan Mitchell but absolutely a worthy starter.