That’s great @sls141, I’ve been able to replicate exactly what you’ve done, and the brightness is now much more uniform across the image 
I need to now adjust general brightness and saturation of the new image, I can merge all the layers and then simply use the HSL tool, but I’d rather do it in a non-destructive way, i.e. don’t merge the layers, and use a new or existing layer to adjust the HSL. I’ve tried doing this with each of the various layers (also with a new layer), but the HSL tool is applied in a a gradient fashion because of the layer with the gradient. Is there a way to apply the HSL uniformly to a layer without the gradient layer affecting the HSL effect?
Thanks again!