If we're wanting to have custom settings (not preset settings) kick in, the SCSS needs to be turned ON?
Hi. Yes, sure. Any settings in the template interact with variables that are in SCSS files. When saving, the SCSS code is compiled in CSS.
For user settings there are own SCSS files. Turning it off, you stop the interaction of settings with the corresponding files. And the code of your site has only that CSS which was activated before the last save.
That is, you have enabled one of the presets and you turned off the SCSS and click the Save button. Thus, only styles from this preset remained in the code. All further interaction with the settings from this moment does not work and does not contribute to any changes.
Disabling SCSS is only necessary if you do not want to use the template settings and want to write only your css code instead. Personally, I prefer to do so because when using the template settings it is not possible to use multiple copies of the template style with different settings.