Hi Mark, congrats on the new scope. As Nathan said a star mask will help separate the nebula and the stars giving you more control as you stretch. Also making adjustments through the channels can sometimes help with improve nebula. As for finding the optimal ISO for your camera try this
https://www.photonstophotos.net/Charts/RN_ADU.htm. I'd say about 400 or 800 would be a good starting point. However, sometimes the recommended ISO isn't right and you may find that 1600 or 3200 is your magic number. The tables are only guides and you may have to experiment a little to find your camera's sweet spot. All that aside, you've got a really nice capture of Carina. There's plenty of information in your histogram and that's always a good sign that you're doing things right. Nice work and I hope to see more.
