My gaming personality is so, so far away from “Completer Finisher” that despite buying a freakin’ console just to play this game, I have never even gotten close to this end-game you are describing hehe.
New thought . . . another “hook” that Brigandine offers is taking a little gang of favourite units / knights / monsters and carefully nurturing them from weaklings into death dealing machines, not just by levelling but also by evolving and equipping. You do not have to do this and it is probably highly inefficient, you just can.
Games in which you can do something similar include Battle of Wesnoth, probably the closest match, also to some extent Fantasy General and Panzer General, maybe Disciples 2? Probably other games I cannot think of too?