I'm not sure what former UK player you could compare Diallo to. I'm not sure such a player even exists. If Rajon Rondo had been 6 inches taller, he might be a decent comparison.
IMO he will shoot around 32% from 3 and 72 % from the line . But I think he will have a game or 2 where he goes 5-6 from 3 ( but also have games where he is 0-7) . He is here to shut opponents down and attack the rim .
I've never seen any info anywhere describing HD as a streak shooter which is what you describe. Only a poor to very average shooter who will never hit 5/6 from 3. Doubting he ever takes 6/7 shots from the arc. I cringe to think so altho admittedly we haven't seen him at all. He obviously didn't shoot it well in any NBA situations. Dribble drive for him.