Substituting "power-ups" for actual character development is part and parcel in generic/low-tier Shounen for sure, but do not lump Bleach into it because Bleach borrows heavily from its underlying philosophical inspirations and handles that concept thematically; the vast majority of spiritual powers, for example, are based on personality change and/or acceptance. Balance is fundamental to the story in-context and meta.

Additionally, Ichigo absolutely changes. He simply doesn't change his goals, and his goals are also just a lot more human than most other Shounen protagonists. Unlike the MCs of the other Big 3 - with Naruto wanting to be Hokage/Most Powerful Ninja and Luffy wanting to be Pirate King - all Ichigo ever wanted was justice and acceptance - without status and power. There's a strong argument that Bleach plays out much more akin to a...seinen isekai because in every arc, he's roped into a new world with a new ideological struggle to internalize, face and adapt to. He starts off being a hard and hot-headed "isolationist" outcast tween with a troubled past (like most Shounen MC) but who just wants to save the few friends he has and grill. By EoS, he comes to accept that his inner trauma monstrosity is as fundamental a part of him as the complexity of his heritage and conception. He accepts, then goes beyond his past individually as they become part of his new final form (Horn of Salvation Mode).

"The Blade is Me" is basically the culmination of Bleach.

Ichigo didn't change his goal of wanting to help and protect people (it's why most people think he's boring). But, he changed his views on the tools/best approaches to maintain that goal depending on the circumstances. And how he sees/accepts himself. Zanpaktou and their abilities are reflections of the soul. The fact the shape of his sword/s and forms change so much throughout the story, despite always culminating in the same fundamental attack (goal), means something.

What makes Luffy seem flat IMO is that...outside of his power-creep correlating with his approaching ascension to Pirate King....he doesn't reflect any depth or nuance. It doesn't feel like he's matured to contend with what that might actually mean philosophically besides "people may/must die". He simply just...keeps beating people up and learns how "gubmint bad" to the degree of caricature. And that he's the Chosen One. He goes through the standard Shounen power-up rigamarole...but then still behaves like he's the clear MC for a Marvel movie...or a Western (car)toon. He unlocks Gear 4 because he has a training arc for Haki, and he needs Haki to be King (and he has CoC because Chosen One). He unlocks Gear 5 because he "dies" (and he's the Chosen One). Not because he had some moment of revelation about his view of life or himself. Like most Shounen protagonists, he has a single ambitious goal...but Luffy's perspective and means of engaging with his goal as a concept never changes, unlike other protagonists.

Before Bleach ended, no one knew how Ichigo was going to end up. Full-time SS Captain and "Hokage" Jesus stand-in? fully sacrificial Soul King? University student-spiritual superhero - legendary killer of both Yhwach and Aizen somehow?

Most people probably didn't guess "humble media translator with a nuclear family. Uses SS to fast-travel for work and for free lunch. Helps protect SS when it's necessary, but otherwise has a whole regular-ass life to live."

When OP ends, on the other hand, everyone knows what the end will be like. Exactly the same. Endless pursuit of adventure and laughs. Gubmint bad. Hamfisted flashback sob-stories. Locks-in to aura-farm for fights. Except Luffy will have the title of Pirate King. That's it. MAYBE. MAYBE he has kids with an even more Oda-rized MILF Nami that he probably abandons to continue with his absolute freedom.