That doesn't work since progression is progression. Sounds like you don't raid at all.
Like if my small guild progressed through normal, we move on to heroic. As we gear up, we eventually try mythic, even if we can only do 2 bosses. How are we placed in difficulty? Are we a mythic raiding guild? Are we Heroic? How does everyone understand what difficulty they want, even? If we want to attempt Mythic by brute force out gearing early encounters, we are now a Mythic guild? And if not are we going to be limited from doing the content or gatekept by server?
People don:t see themselves categorized by difficulty level.
Like if I asked you what your difficulty level was, how can you explain it in terms everyone understands? You only raid Normal and just stop? Every tier is s stepping stone, a level of progression. They are not parallel progression paths. There isn't really 'dedicated Heroic' raiding, it all depends how well a group happens to perform and what their progress is. Sometimes my guild beats Heroic easily and we move onto mythic, sometimes a bad composition or unlucky tank drops handicap progress and we never get to attempt mythic. Every raid is different.
If the game was built so that normal raid -> normal raid 2 -> normal raid 3, then sure, difficulty works. But it isn't built that way, the progression is normal raid -> heroic raid -> mythic raid -> normal/heroic raid 2 -> mythic raid 2.
And population numbers are also skewed, since only 10% of the population does mythic. And even then, it only slightly opens up after they implement catch-up mechanics and start nerfing encounters. There's a huge skill gap between players who can clear mythic during its first month, and players who clear it after all the nerfs are in place.