I think companies would be willing to pay for automation of packaging tools and especially as long term relationships are being built, because that actually increases the overall efficiency.
Similarly, Cachix also identified a problem (a fairly small one for our use case) and that might result in some people to pay for it.
Having yet another CI seems to be a rather risky investment, because CI systems are a crowded space already.
I don’t think anyone should say that you should or shouldn’t do something open-source or not.
Regarding the severely restricted outgoing traffic, this is a serious problem. The days that there was a “single Internet” are long gone. You already have two people here who could potentially be in your target market and both of them say this is a problem.
In short, I would like to see your talents applied elsewhere, but you are free to make your own business choices, of course.