Hi Todd,
I have experienced the same issue my self. I’m going to assume you have separate darks for each different length of sub ? I found on my Canon that anything more than 5min subs introduces issues no matter what the outside temperature is. I also found that I lose dynamic range. I’ve tried so many different combinations of different ISO and sub lengths with calibration frames for each and no matter what I do, I can never seem to get as good an image as i can with just one length and one ISO and just lots more of them. As Raymo suggested and lots of other people do, there is a “ sweet spot “ but it’s not just about thermal vs read noise. It’s also about dynamic range. My canon’s “ sweet spot “ for noise ISO 800 but I lose a lot of colour at that so I’ve settled on ISO 400 and 3min subs. I get very few noise, colour and other issues at this setting but it does mean 10hrs of subs as a good amount rather than 4 or 5. My advice for what it’s worth is to take time to find what works for your setup at a single length , single ISO and quantity. Only use the much talked about sweet spots as a guide of where to start from. Good luck and I look forward to seeing your images
Cheers