Oh, that’s a good point. Your resolution may be higher than hardware acceleration supports.
As for the VBR quality setting of 99%, that will probably be overkill. The YouTube preset uses libx264, which has its own “quality slider” called CRF that ranges from 1 to 50, where lower numbers are higher quality. A value of 18 is considered visually lossless. I use a value of 16 for my “masters” knowing that YouTube will immediately transcode a new copy off of it.
In your case, using a Shotcut quality of 99% translates to H.264 CRF 1, which is super slow and makes super big files without delivering any extra visually-noticeable quality. Shotcut quality 68% corresponds to H.264 CRF 16 if you would like to try that as a starting point. You could raise or lower it from there according to your taste.
Here is a chart I made that lists the correlations between the Shotcut quality percent and the H.264 CRF value: