If you watch Act Man’s review he hits pretty well a lot of the same points you bring up. The UNSC is supposed to be organizing a resistance. We get audio logs that indicate they did, and a handful of the Spartans in the logs are never confirmed dead, Lasky was supposed to head down to the ring, Hyperius has a dialogue (with IWHBYD skull) claiming he let Locke go, but there is not a leader to be found. The world ends up feeling pretty boring when you’re the only one in it doing anything. We go through all the trouble of reclaiming the FOBs for what the Marines to just stand around? The Banished never try and retake them.
As time goes on it becomes more apparent that while Infinite was heading in the right direction compared to 4 and especially 5: the game launched too early and never had clear direction. They did a great job with chief and the weapon but there is nothing else going on which makes no sense. Lot of missed potential. Hopefully they bring more characters back into the story to do some of these things like manage the FOBs, direct the Marines, gather Intel against the Banished.