After struggling for 50+ hours to unlock a consistent swing with a mouse, I stumbled on a few things that finally made it possible. I now feel like I can bump up to "Pro" or "Master" level difficulty and actually have it be a satisfying challenge rather than feeling like throwing my mouse through the wall.

Tested on the following hardware:

• Ryzen 5 3600 on a Gigabyte AORUS 450
  
• 16 GB 3200 DDR4
  
• EVGA GTX1060 6GB
  
• Redragon M801 adjustable DPI mouse (600 - 12,400 DPI)

Here's the steps, they're pretty simple.

• Turn off v-sync in the display settings. Optionally reset the display settings (other than resolution) to their defaults and tweak from there. I ended up setting everything to "High" and the render distance slider to 75.
  
• If your mouse has adjustable DPI settings, set it to 1600 DPI.
  
• Change the "Sensitivity" setting in the PGA 2K21 settings menu to somewhere between 75-85. Yes, I know it seems counter-intuitive to raise the sensitivity, but trust me. With my mouse, I ended up setting mine to 82.
  
• Now go and do the "Swing Calibration" in the Practice menu inside the game. Do as it says and try to keep your downswing timing relatively consistent.

After doing these 4 things, you should see a dramatic difference in controlling the downswing timing. Go hit on the practice range or play a few rounds until it feels natural, and then go dominate your friends and the PGA Tour.