This might be a hot take, but Aizen wanted to lose to Ichigo. Aizen didn’t actually care about taking control of anything he just wanted to be challenged and nobody outside of squad 0 could do it so his plan was to create a being (Ichigo) who could actually challenge him. It’s why he never released KS on Ichigo and never fight him at 100% strength. When Ichigo hit his sword he claimed it felt like Aizen was lonely and desperate for a challenge and as such he let Ichigo be that challenge the same way that Kenny held back fighting at full power. If Aizen wanted to go all out full power, I’m not sure any one individual would even come close including Ichibe but we won’t know until we see Aizen go all out and use bankai and shikai on Ichigo. To some this might seem obvious, but I feel like with non-mega fans I’ve had to explain this over and over again especially is vs battle situations lol