Have say the first 4 dungeons of +10 or higher in a week drop mythic loot. This can be phased in over the weeks in some way. Eg only the first in the 1st week of the season etc…
One of the ways to fix m+ is to make it more rewarding. It’s not that it’s too difficult in isolation, but too difficult and annoying when compared with the rewards.
It’s not worth it to spam it for a vault slot and crests at 10+ - when you can easily out 4/8m and have 4 shots at mythic loot and 2 vault slots.
If m+ was supposed to be challenging and rewarding content for people who don’t want to, or can’t commit to the organization commitment that raiding requires, then it’s fallen a bit too far behind