It sounds like you are not convinced that you really need the battery grip for the Z8. I wasn't too impressed when I saw the design and fit and it was a definite pass for me. Nikon could have done a better job on the design, form, fit and function of the vertical grip. It almost seems like an afterthought.
I can't say definitively, but it seems logical to me that having an L-bracket directly attached to the camera body would be more solid/secure than one attached to the MB-N12 battery pack, which is itself attached to the camera body. Adding the battery pack introduces another moving part into the mix.
So what is your use case for needing an L-bracket? For me, I use them almost exclusively for landscape photography. When I'm doing landscape photography, the need for having a battery grip is greatly reduced (not zero, but greatly reduced). I primarily want the vertical grip and extra battery capacity when I am shooting birds in flight (at 20 fps), or maybe when I am out for the whole day shooting wildlife.
I have L-brackets for both my Z8 and Z9, and I only attach them to my camera bodies when I am doing landscape photography, the rest of the time I keep them off.
Since you are not convinced that you actually need the extra battery grip for the Z8, I would just get an L-bracket that is designed to fit the basic Z8 camera body. Even if you do end up getting the add-on vertical grip, you could remove it and use the L-bracket on the bare camera (for 1500 - 2200 shots
) when you are doing landscape photography.