A good value for GOP is 5 times the frame rate. In this case, GOP would be 125. The longer GOP allows the file size to be smaller. The YouTube recommendations say half the frame rate, but this is unnecessary and offers no visible benefit to the audience on YouTube.
Adding grain to dark areas is a hack that only works for direct playback of the exported file. Grain may work against you when uploading to YouTube.
The idea behind grain is to add fake details that the codec will attempt to preserve, which should avoid big patchy dark areas. However, YouTube recompresses your video to a new constant bitrate file, and not a high bitrate at that. When YouTube sees that extra detail in the dark areas, it will either smear it away back to where you were so it can meet its bitrate cap, or it will try to preserve that grain by compromising something else in the frame, possibly making it look worse than it normally would. Adding grain usually only works for variable bitrate files or very high bitrate files, of which YouTube is neither.
Premiere can export 8-bit or 10-bit files. Shotcut can export 10-bit under certain conditions, but the filters used in your project may not allow for it. So Shotcut will likely render 8-bit and there is nothing further for you to tweak on that point. Also, YouTube will recompress your video down to 8-bit anyway. YouTube only sends 10-bit video to devices if HDR is involved, which Shotcut does not natively support.
Typically, 1440p is the threshold for getting the higher bitrate on YouTube. If your video is 16:9 aspect ratio, then 2560x1440 would be just as good as 3840x2160 in terms of looking better on YouTube. However, the slightly lower resolution would mean faster exports and smaller file sizes for your workflow.