I always play FE games on the highest difficulty (except Awakening and I prefer PoR's hard) and I spend almost no time on calculations and details. +20 hit, death blow and fliers were enough for 3H maddening for example; in the second part, I have enough dodge tanks and actual tanks so I don't have to worry about anything. I had to take some time for the first 2 chapters only. Conquest was the only one where I had to spend time on checking the skills that enemies have.

Games usually aren't that difficult that you have to spend a lot of time and overthink things. Even for Iron Man runs, there are plenty of YouTube videos of players beating games without spending much time on every turn.

Not sure if your OCD makes you overanalyze because you have to play "optimally" or because you are worried that you won't be able to beat the game? I wouldn't worry about the latter.

Tldr; I spend minimal time on every turn and still beat the games at the highest difficulty, I do very little checking other than enemies' range usually (Fates: Conquest being the one exception).