Iris has support for Mythic Mobs 5.

Iris Entities

You can spawn mythic mobs in custom biomes by following this format, More info Entities.
"type": "CAT",
"surface": "LAND",
"specialType": "MythicMobs:JumpingSpider"
The "type": "CAT"does not actually change anything. It just makes sure Iris accepts the entity.
As seen here, to make Iris spawn a mythic mob, you'll need to create a new Iris entity file with specialType, where you prefix the name with MythicMobs:
VSCode will autocomplete Mythic Mob names installed on the server if you are using Iris Studio, too!

MythicMobs RandomSpawns

See the MythicMobs wiki: