please:
-I stopped playing against the AI a long time ago, so the tactics were only tested against human players.

That's commendable!

-on XL with a dungeon, experience shows that the game rarely lasts more than 2-3 months.

But that's questionable! Even against the AI (I play against 7 AI opponents), completing an XL map with a dungeon in two months is difficult.
And playing XL with a dungeon against a human... if it's one-on-one, you might be able to finish it in three months, but how much real time will that take (probably 6-8 hours)?

-there are, of course, such maps, but they are very rare... I think it depends on the map generator or the template by which they are created; I don't know for sure.

Well, on XL, at least one conservatory should definitely appear, but the probability is about the same as finding Utopias and Pandora. In general, random maps are not always a good thing.

-you can almost always handle the minimum, but larger ones are a problem, and with vampires, the maximum is easily defeated without losses (a dozen skeletons don't count).

As far as I know, the maximum is 4 stacks of 50 griffons each, plus 50 Royal Griffons. And how can 12 vampires and various minor units defeat them?
I think that even with 5 stacks of 30 griffons, vampires won't always be able to handle it.