6k is pretty high. are you encoding on AMD?
Discussion
I did the automatic test and OBS set it at 6k. I had to manually set it to 2k for a decent stream.
I'm using Intel Nvidia. The OBS settings say
Video Encoder: Hardware NVENC, H.264
Encorder Preset: P5: Slow(Good Quality)
The OBS wizard isn't the best imo.
I can see how 2k is choppy but on Nvidia 3,500 to 4,000 bitrate should suffice. The AMD encoder is weaker hence you want to push with at least 4,500 if your stream has fast changing frames (like we have in gaming). I use 6k but should be really using up to 10k for the games I'm streaming.
You would probably get away with 3,500 - 4,000 on your connection. Also, if you're native resolution is like 1920x1080 you can set that as your base canvas in OBS in the video settings and set 1280x720 rescaling right below. This should give you a smooth 60 FPS stream.