I have the same issue with Pocketbook Inkpad Color. Using
Night Mode in Koreader makes it less annoying.
I contacted Pocketbook support, saying that the minimal brightness of their device is the same as level 8 (out of 24 max) for Kindle Oasis. They passed it to the R&D department, but who knows if there will be a fix. If you can complain about it as well, this problem may get more visibility.
Here's a good comparison of the readers' brightness. It mentions 6" Pocketbook Color, but it has similar drawback:
https://blog.the-ebook-reader.com/20...-to-brightest/