Edit: Pleaae read the warning posted by Garmin Laurie further down in this thread. The method I describe here may not be safe to use.
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I found an old (thin, flexible) screen protector and out of curiosity stuck it partially on my Fenix 7 Solar Sapphire, half-hoping it to reduce reflections. It worked! Reflections were less intense on the covered part of the screen. (Stupidly forgot to take a picture).
Now it is stuck on fully, and does not make the screen noticably dimmer. I ordered a matte screen protector now and hope that it will work even better. Touch, solar and GNSS still work.
Are there screen protectors with anti-reflective coating out there?