Rule of thumb, always upgrade cpu first before upgrading gpu.
Because
1. many functions of premiere pro are not gpu accelerated.
2. Support for gpu is limited to certain cards.
Very often you will see gpu 0% when doing most tasks, even with 3d elements put in when encoding the video.
For cpu, go for benchmarks to compare.
So when can you start putting money into gpu ? Where is the bottle neck?
Maybe when you reach threadripper level then you might consider gpu.
Even with gpu, you will probably only get 20% utilisation on your gpu whereas you will get 99.9% multi core use with your threadripper.
If so, would it be a good option to get a Ryzen 5 1600 and just keep my current graphics card?