The short answer is either 120 cd/m2 or 160 cd/m2, I don’t recall which. It is a recommended level for my graphics working environment, which I follow the best advice and keep relatively dim. I set it with a monitor calibration device.
The longer answer is that chosen monitor brightness depends not only on personal preference, but also on the general room brightness, any direct light shining on your monitor, and the brightness of the viewing frame (for example my view of this page on pixls has a bright background, too bright for ideal photo viewing). An introductory article: What is the ideal brightness of a photographer’s monitor? | BenQ Asia Pacific
Also, your brightened JPEG has significantly blown highlights in a large area of sky (see black area in image below). That is a bad sign.
So perhaps you are viewing photos in too bright an environment.
cheers