Lots of thoughts on this one.

Everyone is using year 12 pay as a comparison between UA and F9. What you're not mentioning is the huge pay discrepancy for the first several years as an FO. You're also assuming a very short upgrade at F9 or NK. Any of that could change on a moment's notice. The UA upgrade is based mostly on attrition, not growth. Basically guaranteed, long term. I'd say that if you're under 35, or even 40, UA is a no brainer. You'll likely have a shot at wide body captain for your last 5-10 years. Just pick up your things and move to a base. Your life will be a lot easier.

That said, the case for F9 is pretty strong here. Over 45 with kids? Life disruption is a major factor. Commuting definitely sucks. The ULCC's offer an opportunity to make up for lost time, if all the forecast growth pans out. In terms of system wide percentage, you could be in the best position at a ULCC. One more hiccup, internationally, and the legacies could be in trouble. At least short term.

The case for F9 and NK would be stronger with a significant increase to years 1-5 FO Pay. Profit sharing wouldn't hurt, either.

I recently went through a mid life career crisis, too. None of this is easy.