Just look at the Iranian anti-ship ballistic missiles. They basically have a seeker on the warhead which tracks the target for terminal guidance. Sure that's at way lower terminal speeds but I wouldn't be surprised if something similar was doable. Also I read a couple of NASA papers a couple of years back about communications with reentry vehicles. For one the reentry plasma is mostly focused on the nosecone of the reentry vehicle, so you can communicate a lot better with satellites in orbit via an antenna on the back of the reentry vehicle than with ground stations. For another there seems to be a radio frequency window which is mostly transparent to the plasma field. If you transmit in that frequency range you can still get communications to and from the reentry vehicle just fine. So the plasma isn't nearly as impenetrable as once thought to be.
I don't know how the Iranians acquire the target, but in order to have a terminal orientation, you must necessarily have a MaRV.
With regards to methods other than satellites for tracking ships, the Chinese already have drones to do this like Soar Dragon. In 2019 one supposedly tailed the Ticonderoga-class cruiser "USS Antietam" in the Taiwan Strait.
That's exactly what I'm talking about. China has external actuators for targeting targets such as the Soar Dragon, it necessarily needs these assets to obtain confirmation of the target before launching the ballistic missile. A satellite cannot obtain to be a target acquisition platform as well as OTH radars, the ideal solution adopted is precisely UAVs and I know that China is advancing in this field.
However, what I say is that before these vectors were operational, how did the DF-21D achieve the target's accuracy with an incomplete satellite constellation it held and without these external actuators?
Don't get me wrong. China is really doing something incredible in Asia with A2 / AD capable of denying an area for a CSG and is starting to "push" out of the First Chain of Islands, but this architecture is not yet completely complete and I do not imagine that China manages to hit a moving target 1,500 km away. This is changing with the incredible Chinese development that I follow through this website, Soar Dragon itself is an incredible platform for what I am arguing, but that development is not yet capable of overcoming the challenges presented in the western Pacific.