I am not sure why you want to keep 8-bits: you are not loosing anything by casting to 16-bit... and if you open the file with e.g. Fiji and stretch it it looks ok.
I will try to modify my VIs to correctly handle also 8 and 32 bit inputs. Might take some time, however!
...took much less than I expected... 😉
Here is a corrected version of TIFF_writeMultipage.vi. Please use this one: I tested and it does indeed work with both 8 and 16 bits images.
I tried to use the function and it saves multiple images into a single multipage Tiff but in my hands, it saves multiple times the same image.
In case it helps, i have implemented (from the U16 only version sadly), a VI that doesn't use IMAQ but directly a 3D array to save image. I have tested it and it saves the different z stacks as you want.