I doubt you are going to find 'anything' south of Playa that one would even associate with the words 'budget minded'. To my knowledge there is nothing inland between Playa and Tulum. At Tulum one can go inland towards Coba but even if you found something what are you going to do after fishing? You suggested that you have visited Mexico a number of times, mostly (fishing) south of Cancun. I suspect that really means south of Tulum towards Punta Allen for the great flats fishing. So you should know about any opportunities there.
You said 'north' of Cancun but mentioned Progreso and Campeche both of which are actually west and southwest 300+ miles. One thing you should know is that by the time one gets to Progreso, one is completely out of the Caribbean and well into the Gulf of Mexico. Don't expect the waters there to resemble what you've seen south of Cancun. And by the time one gets 'around the horn' to Campeche one is definitely in different waters. That's not to say there is no good fishing around these places (I don't know!) but it ain't the Caribbean. There is an estuary around Progreso and another 40 miles east of there around Dzilam de Bravo. Farther east (closer to Cancun than Progreso) is the Rio Lagartos Reserve. There would be great fishing there Iím sure. In the other direction, 50 miles west of Merida out around Celestun (on the coast), there is a huge estuary some of which is in a biosphere reserve (winter home to 'millions' of flamingos). Lot of fishermen there, but I don't know about sport fishing.
Two other places around Cancun that you may have fished are: directly north in Laguna Chacmochuk where there should be excellent fly fishing. It has easy access from Cancun by car, then boat. The other, less convenient but with great fly fishing, is at Isla Holbox. It is 'around the cape' but too far to get to by boat. The road there takes circumvention but the reward is great. Probably 2 hours + a ferry ride to a virtual paradise!
When people say Cancun they think, of course, of the Playa. And no one would call that budget. But of course there is a very large Mexican town associated with the Playa. Surely one could find a rental there within a budget mindset. Maybe even a gringo or two. Then one would be set to go in many directions to fish and, although none of your posts have mentioned this, have access to a life other than fishing. And with an international airport having many good flights to 'anywhere', you'd be set to leave whenever you chose.
You said, "all I want is a quiet place to rent or/then have my own place, spend a couple days a week tying flies and fly fishing off the boat, & visit my kids a couple weeks out the year". Sounds like Cancun is just the locale for you. I just hope you don't mind the heat! You would rent at first, of course, and when you are not fishing or tying flies maybe you'd find a little place that you could buy.
Check out this link that describes a couple of the places I mentioned.