After more lessons learned, and after relooking at issue after being properly, correctly, asked to clarify, I see this Request for Change is unnecessary.
Request for change
Add a option/button for each mask tool to allow the current settings, be that slider positions, or custom brushes, to be retained as current default if checked.
When the option is checked, the auto reset should automatically be unchecked
When the option is unchecked, the auto reset should not automatically be checked, and settings should not be reverted to true defaults.
For user to change the sliders and retain those as a new default, user would change the slider desired, uncheck the new button, then recheck the button (if the auto reset switched on at that point it would foul things up)
I'm not sure I understand your request. There is a checkbox at the bottom of the panel. If you check that box, then the sliders are reset to zero. If you uncheck it, then the slider settings are remembered, just like you ask. So it seems to me the option is already present.