A Mythic+ dungeon is a much more difficult version of the original, and loot doesn't drop directly from bosses; instead, a chest will spawn at the end containing a number of items that will increase depending on how quickly you cleared the zone. The level of this loot will increase based on the difficulty level (Keystone) of the dungeon you completed, starting at 345 and scaling to 370 for a M+10 or higher.
I highly recommend actually staying in the 20-60 range zones until you ding 62. The reason for this is that despite getting a slight penalty in experience for being over-level, the advantage of having flight will more than offset this. That, and WOTLK and TBC zones are the slowest part of the leveling process. Minimizing your time there as much as possible is worthwhile.
The reason is because higher rank recipes stay orange longer, so you get guaranteed skill points longer. The rank 2 of this recipe turns yellow at 135, while the rank 3 is orange up to 145. The rank 3 also cost fewer materials, so you will save a lot of materials by getting the rank 3. You could actually level to 150 with the rank 2 recipe, but it will be green for the last 8 points, so you will have to spend a lot of extra materials, and there is no reason to rush 150, so I recommend to wait until you get the rank 3.
Despite all of these problems and this laundry list of glaring systematic issues, the crowning achievement of Battle for Azeroth’s endgame is that it still remains enjoyable all due to those two pillars of raiding and dungeons. That won’t last forever. As the expansion goes on, and the grind necessary to progress becomes even more demanding and the time spent in content that simply isn’t fun increases, it will be harder and harder for raiding and Mythic+ to carry the expansion along.
Tiragarde Sound and Stormsong Valley are both fairly similar zones, but Stormsong is a little longer, meaning that you might not even have to finish it to hit 120. Tiragarde also comes with a bit of a warning. There are plenty of side quests to do in the zone after you finish the story quests. Don’t do them. It’s not worth the time or the experience you’ll get.