Try this:
Take it to the max speed then export it using HuffYUV or Ut Video from the lossless preset. Make sure you have enough space because lossless exports can be big files depending on resolution and duration but they won’t lose any video quality when you re-encode them which is why I would suggest it here for this case. Then take that newly rendered lossless file and bring it to Shotcut and speed it up more. 