Nate Payne published a great document on how to fix the washout with the Beta75X quad.

Basically, according to him, all you have to do in BetaFlight is the following:

  1. Enable "I Term Relax" (a new feature in Betaflight 3.5).
  2. Leave the "I Term Relax" options at their defaults. This helps to reduce i-term blowouts
  3. Lower the Yaw P to 65 and increase Feed Forward (another BF 3.5 feature) to 96/96/100 for roll/pitch/yaw (respectively). This preserves the same responsive feel but reduces oscillations!

He did also a good video about that:

And he did also a good writeup with lot of details !