I gave a few thoughts on brightness in the middle of this post, link.
I don’t want to give the impression that there is an absolute right and wrong about brightness levels. It is obviously and ultimately a question of personal preference and interpretation. Although, having said that, if large areas of your processed file are clipping in one of the colour channels, that is a bit of a warning sign.
One can have a calibrated monitor from the shop, but if one is using it in a brightly lit room, one will tend to brighten the photos quite a bit. Conversely, if the same monitor with the same calibration is used in a pitch black room, one will be turning down the brightness quite a lot to make it “look right”. The same goes for the on-screen wallpaper or background that might be visible when editing photos: if it is very bright or very dark, it will affect what do you think looks right. However, that is why there are guidelines about the room illumination levels to use while editing photographs, and also for the grey levels on the screen backgrounds.
It is there in RawTherapee for me, under Lens Profiles/Manual/Camera=E-M5/Samyang/7.5mm ƒ/3.5 UMC Fish-eye MFT. I can’t recall if it is there by default, or if I found it somewhere when I started using that lens about 6 years ago.