I previously had a S9 and now I have an S21. In both cases, I think the Always On Display needs a 'Night Mode' setting that would change the AOD text to a deep shade of red (according to a schedule). This was a feature I had on a previous Microsoft phone that was great.
The very light color options are too bright, especially when sleeping. The palette of colors available to the AOD app are VERY limited. Human eyes do far better at night with the red end of the light spectrum. Currently I use a piece of red acrylic which I put over the screen.
This cost of implementation is super cheap, and would be really helpful.