Thank you, Nosferatu!
I was thinking wrong and making life hard on myself.
Now I feel kinda stupid, but it isn't the first time.

After upgrading a large Factorio 1.1 save to Factorio 2.0.73 with the Space Age expansion, progression appears to be blocked at the point where travel to other planets should become available.

I have researched far enough to require travel to other planets and am able to:

Launch rockets normally ...