Hi sleepypeepy,
As you probably know … sateen and percale are weaves that are usually used in reference to cotton but can apply to other materials as well. Sateen is usually considered to be softer than percale which has a more “crisp” feel.
It may be worth considering higher grades of cotton (such as Egyptian long staple or Supima) which are generally softer than cotton that has shorter fiber lengths.
If you’ve been testing cotton sateen and percale sheets it may be worth trying various types of viscose fabrics such as bamboo, micromodal, lyocel etc or cotton blends of these which tend to feel softer.
Cotton flannel or cotton jersey sheets may also be worth considering and are softer as well.
While it’s certainly not my area of expertise … post #7 here and the posts and information it links to also has more about different fabrics and sheet options that are available
Phoenix