How old is the toilet.<<<<<big question
1)Do you use puck style bowl cleaners that drop in the tank. Sometimes they will move into the flush hole under the flapper in the bowl causing a water restriction.
2)Lime build up over time will trash it. No amount of muratic acid will clean it out.
Could be something stuck in it?? but you would need a Ridgid or General toilet snake
Now as far as the subfloor and visible water.......your wax ring is shot, maybe among other things.
What I do is pull toilet.... replace guts(new Fluid master and korky flapper),wax ring, use johni-bolts(saw them off and new caps).
repair subfloor if needed, so toilet doesn't rock.
other than that you probably need a new toilet.
I have repaired/replaced hundreds of toilets
