Nothing will help at this point. The tree can live decades like that.

There are little tiny microscopic bugs living in the hole. Should I try to kill them or are they harmless

I've heard to use 10% bleach water to help kill rot and not let it spread as fast. Any truth?

The only thing that will stop rot in a cavity like that is the trees natural ability to wall off the infection. The bugs aren't adding to the issue. Even if you cleaned out the dead wood and filled the hole with some material, it won't help the tree.

Hey Jimmy-

It looks like you are almost there. How much further does the rot go into the collar? If you take another inch off would it be all clean wood?

I don't think so. The visible rot inside the hole goes probably even with the trunk and I'm sure it extends a tiny bit past what I can see