I have half a mind to call out Daio on his bullshit and ask him why he thinks Aizen has a personality. But he'd put up some sort of trolley defense like 'his pride is his personality' or some bullshit like that.
Because let's be real here. Aizen is a shit character.
And I say this as a liker of Aizen. I loved his attitude after he reveals his true nature and during the FKT arc.
Aizen is this seemingly giant titan sitting on a mountain of power. Coming down to crush the characters like flies. Utterly prideful and sure of himself. Seeing him easily defeat Ichigo showed just how powerful he was. Him forcing Hitsugaya to stab his own best friend showed how evil he was. Which makes all the more satisfying when Ichigo fucking stomps the man when he achieved UNLIMITED POWER with the hog.
As a villain he was great, and he had a lot of charisma.
As an actual character however, well
@TrueG 37 said it best. He has less personality than a plank of wood.
Like the only traits Aizen has shown for the entire manga are pride, and smarts. Which he uses for trolling.
His goal is to become the Soul King. But why? Why does Aizen want to become the Soul King. What is so important about that goal that he's willing to kill entire cities for it, research forbidden topics, and manipulate everyone around him for thousands of years? Nothing. No idea, the only thing we know about Aizen other than being an all-powerful troll is that 'he was lonely because he was so powerful.' Which is probably just Kubo realizing he made Aizen too powerful and needed an excuse as to why Ichigo defeated him. Because not only does it go against his seeming characterization. But also because if he really wanted a equal well there are multiple people in the Gotei 13 that could've filled that role.
Aizen is such an incredibly bad character that he doesn't have a backstory, like nothing on how be become so powerful just hurr-durr I troll, kill, and created Ichigo.
Yhawch is worse. Because not only is he just Nazi Aizen. But he had the actual makings for a backstory.
Like we know that Yhawch created the Quincy, and was treated as a messiah-like figure(the name Yhawch is literally based of Yahweh) and Hollows started killing Quincy so they started killing them back. Then the Shinigami come in and are like "Hey bros, I get your angry that your friends are dead cause they were killed by soul-eating monsters and their souls won't be released until the hollow is dead, but you gotta stop killing them or else the balance of souls will be fucked and Hueco Mundo and Soul Society will smash together with Earth."
The Quincy don't listen and keep killing Hollows, so the Shinigami start fighting the Quincy till nearly the entire race is extinguished and Yamamoto kills Yhawch.
Like you could do something with that!
Maybe Yhawch wants to become the Soul King and erase death because he wanted to see his old friends again. Maybe he wanted pull a Fate and thought the best way to give humanity salvation was to erase death and then Ichigo can go "It isn't your place to decide who gets salvation". Maybe he wanted to become the Soul King because he hated how the being had all this power but created this fucked up system. Maybe he was just a conquerer that loved conquering but realized that if he died he can't conquer anymore, so he wanted the soul King power to conquer everything and live forever as an immortal tyrant. Maybe he just had a hateboner for the Shinigami for killing the entire clan of people he created.
Maybe he just feared death.
Long story short tldr. Aizen is a good villain but a shit character that you can't even really expand in fanfiction because he doesn't even have a personality or backstory. While Yhawch is a shit villain and shit character that however, at least had enough information about his and his clans past that Kubo could've made an at least cliche backstory. But nope instead just throwaway the potential and make redux Aizen but now a nazi.
Edit: Also he copied Aizen's entire 'I created you shtick' with Ichigo. Which was already bad for Aizen.