If you’re on the Advanced Export menu then the resolution and fps there are the settings that were set for your project. Fps has nothing to do with video quality so taking a 30fps video and exporting it as 60fps to try and improve video quality is pointless. It’s just going to make your file unnecessarily bigger because you will be exporting double the video frames. Also, doing that ruins the video because you will not be exporting new frames. It will just copy the same frames 2x in order to get to 60fps so you will not get the 60fps motion that you would from an authentic 60fps video.
In general, there is no reason for you to be touching B frames, fps, GOP and all that. Shotcut does not put bad settings by default. The default settings in the Stock category are all of high quality.
If you really need the “best quality” for video even though the default quality is “high quality”, then go to the Advanced Export Menu and in the Codec tab change the quality to around 65%. Anything above that as @RilosVideos explained is not going to be noticeable. But again, the default of 55% is already at high quality.