But just look at the lead up to the war. Ace didn’t have the mentality of a King /Conqueror. He had pretty well accepted death and thought his life was one that needed to be extinguished.
Though I'm not sure we know how much proficiency he has with Haoshoku. We know that he has it, but can he control it at will? It would be strange for him not to be able to, but we can't be sure.
I'm up to the point in Wano where Luffy says he can't use Haki with the seastone cuffs. I guess it's possible that Ace could use Haoshoku Haki naturally.
Yeh the seastone makes you tired so it could have stopped him using it, especially if he wasn't that good at using it. Luffy also didn't get good at using his haki until he was Ace's RIP (I wept) age and that was with 2 years of dedicated training, which Ace maybe never got since he was with WB's crew the whole time and going on missions.
Considering Marco commented that Shanks used the same type of Haki as WB when knocking out the fodder with Haoshoku rather than bringing Ace into thee equation too, it seems very doubtful Ace could use it at will. probably never developing it further past his childhood. The only source that says otherwise are video games such as Pirate Warriors 3, which are far from a solid source.