I kept messing with this hours and hours and finally got it working in my Tab A 10.1 2019. I believe that the biggest issue was adaptive battery feature that doesn't work properly. In addition to delaying notifications it noticeably slowed down the device, which could explain why some people report disabling doze actually increases battery duration or why some report disabling all Google's services as solution (these "smart" AI-things are probably among Google's services). In addition to 1.) disabling adaptive battery, I have 2.) disabled "optimise settings" for battery 3.) disabled app specific battery optimization for gmail. Not sure if 2.) and 3.) are needed and I believe the main problem was 1.). I might have also turned off other AI-based battery optimizations I don't remember Hope this helps someone and and saves nerves.
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