It was a pleasure to make this workflow, I hope it will help with your data processing ;-)
Yes, I think you need to increase the exposure time of your subs to get better quality. The lokation (background) of your data is at 0.01 or 1 % of the data range. This is very low. You can easily make 15 minute subs. The read noise will be less of a problem then I think.
It’s good to realize that this is narrowband data, so read noise has a much larger influence than with regular RGB data, where the read noise is more easily swamped bij the shot noise of the sky background.