WoW isn't the kind of game to benefit a ton from high refresh-rate. I play the game on a 2560x1440 screen at 165Hz and the game, even with my 1080 Ti on Preset 7 won't ever hit that max refresh, and sure as hell won't do half of that number in raids. Screen real-estate and visibility is better for WoW so get an ultra-wide(ideally 3440x1440 34").
In my honest opinion though, go and build a small form factor Desktop to keep in your room as your sit-down at home setup and the macbook for work and on the go. Node 202 is a fantastic little case for ITX builds as the footprint is quite tiny, not much bigger than an EGPU box. If the monitor supports Thunderbolt 3, you can dock the macbook for when you need both the screen space and Mac. When you're talking the cost of a Thunderbolt 3 enclosure, you're better off building a desktop at that point since you'd get PCIE 16 which will allow your GPU to work at 100% whereas Thunderbolt 3(while fast in it's own right) is a big bottleneck.