Another option is to also use one of the before/after frame **without** lighting (or to also combine all the frames in darken only mode) .
You can then subtract that frame from your lightning frames(*), and this leaves you with the lightning only. Then once you have these (added with lighten only mode) in a group, you can duplicate the group for added contrast, and put it over a darkened version of the lightning-less frame (which is much easier to work with because you don't have to protect the lightning).
Final note: its likely useful to use high precision mode for all this.
(*) ofn-interleave-layers can blend a layer against all layers in a stack with any blend mode)
You can then subtract that frame from your lightning frames(*), and this leaves you with the lightning only. Then once you have these (added with lighten only mode) in a group, you can duplicate the group for added contrast, and put it over a darkened version of the lightning-less frame (which is much easier to work with because you don't have to protect the lightning).
Exemple above obtained by artificially removing the lightning from one of the frames. Also cheated a bit because the top of the trees is reflecting the lightning.
Final note: its likely useful to use high precision mode for all this.
(*) ofn-interleave-layers can blend a layer against all layers in a stack with any blend mode)