It's possible to add Mythic+ level to show only groups for that specific level and not +- 1 like the default blizzard interface does?

Not possible. Since the Battle for Azeroth Prepatch, the group name and comment are protected by Blizzard and can not be evaluated by any addon. See #136.

If you want a single key level only, do the trick mentioned in the FAQ and search by e.g. 15 16 17 to get 16 only.