Your GPU is fine and exceeds the minimum requirements for Shadowlands. With the new cards being announced shortly, I would not buy anything until we see what’s coming. If you really want to upgrade (because you certainly don’t need to upgrade), you can make a decision after the new cards come out. I would also wait and try your card with Shadowlands and see if you’re happy with it.
Does your monitor support 144hz? If you’re getting 125-200 FPS but have a 60hz monitor you can just cap your FPS at 60 and be happy. 
More info about the new cards here: