In iOS 15, I have a primary iCloud account on my iPhone and iPad, but I also have secondary accounts.
Those secondary accounts that I have from a previous iOS version are already configured as desired, but now that my mother has passed, and I inherit her email to settle estate, I can't rename the description to something shorter than the whole email, removed emails are sent by default to Trash rather than Archive, etc etc.
I can see these settings on my primary iCloud account and non-Apple emails through the Settings > Mail > Account > iCloud Account > Mail > Advanced.
On secondary accounts, the Mail list item is a toggle, not a settings screen. I cannot find where the detailed settings would be for newly added accounts, nor any control to change settings on those accounts which were imported from previous OS versions that once used to surface these settings.