Regardless of the PC system you have, you cannot achieve 144fps on PUBG on max settings at 1080p. PUBG is a very good game but it is very poorly optimized. I run 1080ti in sli and I still have frame drops. The good new is the game is very popular and the optimization should improve moving forward.
With that being said, I would buy the GPU first, before I worried about the CPU. For gaming you want to spend you money on the GPU. After you get the GPU see how it plays, if you want more performance, then consider getting a new CPU. I would not upgrade the CPU before the GPU as the CPU is not as important as the GPU in gaming.