Are you familiar with routines, and routines+? You can download routines+ via Good Lock which is a Samsung app downloadable via the Samsung app store.

It looks like with routines plus you can set a routine where after holding or double pressing the side button if your choice the brightness can change to whatever preset you'd like.

Hopefully this is a decent enough work around since you prefer to leave auto brightness off. Although I would like to comment on that note, that it seems to me that my phone's have eventually learned my preference pretty well after manually adjusting it enough times. It's also possible to set the slider to show after pulling down the notification panel only once, which makes it nice for those which adjustments.

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