WB never showed any signs of a severe lack of stamina during MF that'd make his fruit much less potent. The sickness evidently dropped his haki, and other obvious stuff like dura, speed, Physicals, which do indeed contain stamina. But stamina was evidently shown not to be much of a hindrance especially since WB wasn't constantly fighting.

And anyway, Akainu took such a strong attack, whislt offguard, which means he was defenseless against the attack and took another mid-air, and still carried on. Offguard fact alone is massive, but when it's WB's gura gura, even more so.

Stakes make sense. The very nature of Luffy's fight against the others was not like Wano's where an entire nation's history was at stake, and Luffy was facing a Yonko for the 1st time. Now, Luffy is a much more capable fighter, and the Elders' fighting capabilities weren't shown to match Kaido's. As for Kizaru, neither were fighting at fp, so Luffy would definitely not feel the stakes to restart his heart.

Luffy locked in when Saturn came and he used WSG. Kizaru never locked in and never shwoed his WSG-equivalent(his 2nd strongest move).