Wow!
Looking good!
I am a believer that mosaics are better than throwing weight of data at a single panel at times. Say - a 2x2 panel mosaic with one quarter of the data thrown at each panel. Each panel looses SQRT(4) = 2 SNR for the same amount of time, but if you have 25% overlap you crop the bad corners anyway. Another thing - the stars are much smaller in the final image. Also, you can be slightly more aggressive with noise reduction, given the extra resolution (which can be selectively done).
On the other hand, gradients and trying to colour match panels can be a pain in the arse. $%#& I know! Airglow variations being one of the biggest culprits for me.
The most important thing is that each panel gets the same treatment with exposure and processing.
Do you plan to run more panels? Looks like a good project with awesome results already!