Mythic+ is very casual friendly. Not only do you need considerable fewer people than with a raid, you can also do it much faster than a raid. You just need to find 4 people of the same mind-set as yourself and you can play at a level that suits your interests/experience/skill-level.
Don't confuse casual with unskilled/uncompetitive. Casuals can be very competitive and skilled. Don't skill-shame casual players.