Here is an idea for some changes. The idea is that the switch layer has two modes. Normal and FBF.
The adding or subtraction of frame numbers should be just clicking under the last frame number. Once for continuing last layer, twice for a new layer.
I think that after 4 or 6 frames the next box would have a jump to the frame just before the next layer. So insted of say one hundred boxes with numbers it would go like this:
layer hair_1
_2
_3
_4
_99
layer new hair_100
So from hair_1 to new hair_100 is 99 frames.
But you still have vector keyframe (key pose) layers, which would suffer the same serious performance hit that switch layer FBF attempts currently do. Your null layers are just a different depiction of the frames between switch keyframes. It is the key pose layers that are the issue.
But no harm in throwing idea out there. Perhaps AS performance will eventually rise to meet this need.
Why? I have setups with 100, 200 or sometimes 300 layers.
Experiment: duplicate the opening project layers. About 30 layers five times. Place them into a group. (120) layers Copy the group five times. Place them in to a new group. (600) here is where the program locks up on my computer, just stops responding.
So 600 layers is the max? But I'm talking about pointers not layers. So 500 layers and 2000 pointers or null layers would start to strain the system. Of course picture layers would lower these numbers.
Performance is also very dependent on things like how many points are on each layer. An empty layer doesn't have near the impact a heavy artwork layer does.