Maybe the solution would be to turn off syncing and make it „Manually” when you open email app or ”Automatically” to do it only when on charger (details below)
?
I have the same usage as you and Mail app is not even showing in battery stats.
These are my settings on iPad Pro 2018 11” iPadOS 17 :
(using Gmail as main account + secondary iCloud for spam and syncing)
In my case I barely have it on charger but maybe playing with fetch - „Manually” might help there - then if needed you could just open Mail app and pull down to synchro If I remember well.