Why would you export to HuffYUV and then upload to youtube? youtube is going to encode it to an h264(maybe h265 at this point) file for playback. If you upload h264 they’re not going to re-encode it other than to make lower resoltion steps available. Exporting to Huff or similar is only something you’d do if you intend to use the clip in other videos(I did this when I made a small title animation for youtube videos)
If you want to use HuffYUV in your wokflow for performance reasons(which is what I do) the workflow would be
source>HuffYUV>shotcut>target export(usually youtube)
I have found that this(or substituting UTvideo for HuffYUV) has made working with 4k a lot easier without resorting to proxies as moving to HuffYUV takes the interframe compression out. You can make your audio a faster format at the same time(I have to look up what @shotcut said was best but if I recall it was some form of uncompressed audio) you can use tencoder or hybrid for conversion outside of shotcut they’re both just ffmpeg front ends and work well.