How to make all layers the same exposure in PS

I agree that the best approach is to do this, on the raw data, in Lightroom. Despite the name, this has nothing to do with exposure which only takes place at capture. You are altering brightness not exposure.

I also agree moving forward; you now do this at capture with actual exposure as recommended.

Perhaps the better solution would be to use fixed settings on your camera when taking the images. Assuming your camera allows this, set it to manual and dial in a specific Aperture, Shutter Speed and ISO. This should give you a series of images with consistent exposure. I'm happy to dive in to more detail on this if needed.

Although I'm unsure about Photoshop, I know you can do this in Lightroom if that helps at all.  Just go to Settings > Match Total Exposures.  Make sure to select all the photos you want to match and the exposures will be matched to whichever photo is active at the time.