The Dell G3223Q is an excellent option. The G3223Q is a 32-inch, 4K monitor with a maximum refresh rate of 144Hz and support for AMD FreeSync Premium Pro (Nvidia G-Sync also works, unofficially).
At the same time, panels like the BenQ Mobiuz E3210U may end up dropping under $600 again, providing a premium 144Hz punch for much less than new arrivals. The least you'll pay for a 4K monitor ...