I basically just did the dungeons whenever they had a whisper on them. A few each day whenever I had time.
But the last 15 or so I just bit the bullet and grinded them out while watching TV on the side.
As for a reward.. Just a feeling of smug superiority and very limited bragging rights.
Nope. No reward, not even an achievement. Nothing. Honestly a bit of a slap in the face. Especially since they couldn't even be arsed to add a questlog for already finished quests. Took me several days to track some of the last missing ones down. Definitely not fun, but probably a good indicator of how much of a chore the FOMO pass will be.
I'd hope there's at least an achievement. I was disappointed there wasn't one for getting max renown rewards.
I'm still nowhere close to finishing all the side quests and dungeons. Once I get to WT3/WT4, there just doesn't seem any reason to grind through all the regular dungeons.