While this income stream dried up when Legion came out in 2016, Draenor garrisons multiplied the total amount of gold in circulation many times. During the Mists of Pandaria expansion, you were rich if you had a few hundred thousand gold but, after Warlords, rich meant being a multimillionaire. However, the impact of this extra gold was blunted for a while, because WoW Tokens were redeemable only for more game time until February 2017, limiting how much gold rich players were willing to sell.
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.
Congratulations on reaching Level 120! There are a myriad of new things to do. The first thing you should do is head to your faction hub ship where you have been doing your mission table quests and footholds to pick up and complete the quests necessary to unlock World Quests. World Quests will provide you an opportunity to grind reputation levels, acquire gear, and gather Azerite. There are tons of new things to do that we will cover in our Level 120 Battle Guide found here: Level 120 Battle Guide