I've been chewing away at this for quite a while now, an eternity in my attention span. I was determined to combine the mosaic in PI as a learning exercise and had been at it, in vain, for a week. I've concluded PI is woeful AND slow at mosaics. In contrast APP knocked over in 5 minutes the bit PI choked and failed on for many hours at a time.
I have a new adoration for the CEM40ec, I threw out 2 subs (for wonky stars) in the whole project, ignoring clouds and other light problems. Floored.
https://telescopius.com/pictures/view/160060
CEM40EC on the CF tripod
Eon85 + 0.8x reducer
ASI2600MC + SV220 NB filter
2 x 1 Mosaic
360 x 300s (30hrs) 100g -10C 2x Drizzle.
NINA + APP + PI
Not happy with the colour but at this point I'm over it.