Thanks @rfc2822, it helps a lot ☺
I managed to sign in with DAVx⁵ using the app password.
Contacts synchronize correctly but aCalendar only wants to sync my main gmail account.
My wife’s gmail account is “forgotten”: it is not in acalendar’s list of calendars, yet it is duly added and associated with my main account
Why such a restriction? Is the only solution to sign in with DAVx⁵ to my wife’s gmail account?
This solution is not very privacy friendly, it seems to me.