This character never appealed to me tbh.
I would care for and appreciate Sabo far more if he had his own distinct personality, much different of his brother's, and as you said had genuine depths and flaws.
But no he's just a carbon copy of Ace in Alabasta.
I liked he seem to have a sort of psychotic/mentally unstable side to him when he was crushing skulls at Dressrosa. I would have liked him to be more of a gray area, extremist type character, savage and even spiteful, a sort of Punisher-esque or even Butcher from The Boys (who is a parody/reference to Punisher too) type character.
But like Yamato, his character is just shoehorned in andthus feels unnatural. I wanted him to be alive somehow, for Dragon to have saved him and I'm glad he was and is but it's caused such a mess narratively and I really don't like him coming back to just literally replace Ace. Its the same kind of BS as a death fakeout but worse in some ways too.
I loved how he fought with the haki dragon claw stuff and channeling the mera mera through his pipe weapon and such but his character is pretty lackluster, just like Ace himself. Just another rule of cool, good looking Gary Stu, a milquetoast character even.
I just find it so weird he was introduced in the same arc as Ace dying if I remember right and we're meant to believe Luffy has known him for most of his life now, since childhood?
Now we're getting similar shit with Luffy and Uta in FIlm Red and it's just making me wonder if they're purposely trying to take us for fools or they're getting lazy. Oda and the writers of Film Red or whatever I mean. Although Oda is pretty much overseeing the whole film isn't he? Or at least a large chunk of it?
And yes, I did suggest Sabo should be more "edgier" unironically. I think it would have been really good if handled right, it would have been very fitting/understandable. Sabo could be showing signs of major PTSD and grief/regret and such, beyond the grief/regret he showcased before.
I do like how vicious he was vs Bastille and Burgess and I was even a little concerned for him. It felt more realistic, mature and thus believable for him to behave and react like that. I do kinda wish he had gotten the kill on Burgesss too honestly but at the same time I do like Burgess and want to see a lot more of him, especially a big comeback for him, with a powerful new DF too, ideally the diamond/Jozu's one too imo.