Something is not right. I do not remember when I did level and compass calibrations last time, I just place the S50 on the ground, I point it to a major star and let it do the three-step dance followed by the dark image calibration and I am good to go. A very good stacking efficiency is in the 75% to 80% range. I normally achieve 60% or more in simple exposures and 30% in mosaic mode, which is why I do not use mosaic mode. You have 15% to 20% efficiency, that's not good at all.
I only use it in stargazing mode, no planetary or Moon, Sun images, you do that once and then you move to more interesting stuff. Oh, and I am in a highly polluted Bortle 8 or 9 zone.
Start with a 5 minute exposure on something simple, like a globular cluster or even M42, where 15 minutes are more than enough to get a decent image. What kind of staking efficiency do you get in that case?
