Only very recently resubbed, so not dipped my toes into M+ yet, but in each of the 3 prior expansions, my enjoyment of this system got progressively worse. I notice also several threads complaining about M+ leavers.
Scrap the timer from M+ up to the gear cap, have the ilevel cap be around the difficulty of +10 or +15 in Legion and let the competitive players deal with the timer AFTER the gear cap. Maybe they get more drops from end of dungeon, maybe they get cosmetic rewards, but maxing your weekly vault, given it is a single item a week, really doesnt need to be that hard.
The timers for casual players, particularly in PUGs, is not fun. It means you cannot replace leavers, it means there is never any kind of banter or social element, because that is inefficient and wastes time. It also means the only moment of excitement over gear is once a dungeon at the end. In BFA and Shadowlands, my best time running a dungeon was week 1 M0s, before they became trivial and when you cared about the gear. There is no reason why you cannot have scaling difficulty without a timer.
Without the timer, you can replace leavers. You can have a moment of excitement after every boss kill, not just potential disappointment at the end of the dungeon. The overall experience is less stressful. You can also scrap tyrannical and fortified, which are pretty universally disliked modifiers.
Instead, just have a weekly lockout for 3 levels of mythic+, each tier with an extra affix with end of dungeon rewards for 1, 2 and 3 affixes being equivalent to LFR, normal and heroic raids respectively, and your vault rewards being 1 tier higher based on the dungeons you ran.
This will also encourage a larger variety of dungeons being run due to the weekly lockout, not just a focus on which dungeon is most efficient to run once you have your score high enough. On that subject, there would be no need for raiderdotio or its equivalent in game. This toxic elitism is obviously a consequence of the timed nature of M+ and the inability to replace leavers. Players are not prepared to take a chance on someone who might fail.
Timers and key pushing should be for those who enjoyed challenge modes. There is nothing wrong with this existing, but the large majority of people were not interested in this. Why impose this style of gameplay on the entire playerbase?