Earlier quoted context omitted.
Can't you get certificates by doing DNS challenges and use those certificates internally? If you don't have to be completely airgapped, doing the DNS challenges shouldn't be too hard.
It is my understanding that DNS challenges are discouraged and/or being deprecated due to the challenge results being less trustworthy than more stringent verification methods. There is also the operational overhead that arises as SSL certificate lifetimes shorten; It is my understanding that there is now a case being made for SSL certificate lifetimes shorter than 24 hours.
And also the lifetime isn’t a problem in the setup I described, the internal server that uses the cert can do the dns challenge so it can get a new cert whenever it wants. It only needs to be able to access the DNS api.