Dragonflight Keystone Master: Season Four

I was able to complete this with 6 M+6 dungeons and 2 M+7 dungeon.

Mind that you get less rating for the same level dungeons this season. Last season to get, for example, 312 rating for a dungeon (which is about what you need to get Keystone Hero), you'd need a 17 and 18 timed on two different weeks (confirmed by multiple posts on Wowhead under Keystone Hero achievements and from personal experience). Now, having two 8s timed on two different weeks or even 9 and 8 gives you less rating. This is in complete contradiction with what Blizzard said in their content update notes, here I quote:
- "Dungeon ratings should be equivalent to what they represent in the current system."

I refuse to believe Blizzard did not know that you'd get less rating and chose to word this this way, I just don't know why choose to lie instead of straight up saying "you get less rating for the same dungeons as we want Keystone Master and Keystone Hero achievements to be more meaningful".

So yeah, you will need higher tier dungeons timed this season to get both achievements. The rant is just to explain that I didn't pull this out of nowhere.

Your second statement is not true. Last season I was able to get this on multiple characters by doing 12-14s which translated would be 2-4 keys now.