There is already an article on this topic, but it is from 2019 and perhaps something has changed.
Is it possible to combine the image layer with a fill layer? So that both levels are processed at the same time?
I have 3 layers with the same values, but I have to run them all one after the other and that takes a really long time. If I could combine the 3 levels it would be a massive time saver.
The only way to process the fills and the bitmap together at the same time is to put them into one layer with the same settings. It might be difficult to find common settings that will produce the same quality of output.
To do this, use Edit>Convert to Bitmap (or available from a right-click) and convert the fill layers to a grey bitmap. You can then use Tools>Adjust Image (also available from a right-click) since you will probably need to increase gamma to reduce the dithering effect, now you can change this layer to merge it with your other images.