Oh, now I understand better the whole picture.
If the uneven background is the issue, please consider trying the module Rescale with the following settings:
It then (hopefully) will stretch each image to a similar range of background/foreground and then you can use Adaptive Otsu to do the segmentation, and don’t have to bother using manual threshold for each of the input image. That method is a bit too laborious, no?
(*If you want to measure object intensity later, do measure the intensity from the original image, not from the stretched one).