Allow to allocate additional memory (preferably do output by fixed square regions to avoid memory consumption) while combining final image color from layers and groups Mode blendings.
Enable blending Mode option for Group layers.
Do all blending operations for sublayers of Group layer inside it i.e. recursively (only use extra memory if required).
Result: This will allow to use Addition inside group to sum different lighting (which is physically correct) and then apply it to base image with Multiply (which is also physically correct). And do other tricks.