Concrete or gravel for deck post

You did not say what kind of the wood the deck is made of and the condition (other than old). I have completely rebuilt some 30+ year old redwood decks in the past that were nailed with a nail jack like this. The boards bay be able to be turned over and an almost new surface is possible after a little sanding. if the surface has been well maintained and the nails are pulled without damaging the surface of the wood screws could be used to replace the boards in the original positions. this will take more time than replacing the decking but can save thousands. You will also find material cost has dramatically increased and the quality of the lumber is lower today than it was 20+ years ago. it all depends on if there is any rotten ares with the decking if it can be reused redwood and cedar have very long lives compared to other soft woods.

I say the 30" depth of post hole is adequate for said fence with given construction method; no need for additional support.