Sorry you are having this problem, I've never experienced exactly these symptoms ... but the rear drums have caused me plenty of repair time over the years. I do my own repairs and try to stay on top of issues, but can say that I have spent more time than one would think necessary on the rear drums.
I use a synthetic brake fluid and try to bleed the system annually. It helps that my vehicle sees little road salt or other rust-makers. I use the looseness of the parking brake to indicate that rear brake service is due. If the parking brake handle must be lifted high to get sound holding, its time to remove the drums for a look-see-adjustment.
My experience has been, the self adjusters work well for a period, and then manual adjustment is required for good braking.
I remove the rear drums and thoroughly clean and inspect. I remove all parts from the backing plate, and sparingly lubricate all friction points w hi temp grease. I usually use a kit for all springs and hardware. The drum shoes are reused only if they look very good with plenty of wear material.
I always replace the cylinder assembly. Not only do I replace, but I disassemble and lubricate the new piston cups and springs with hi temp grease. I find that a sticking piston cup is the beginning of drum brake problems. One piston cup sticks/freezes, and the other cup is left to do all the moving. So I make sure that the piston assembly is easy to slide ...to and fro... in the cylinder before installation. If you buy new cylinders, be sure that they give you the correct diameter cylinder bore!
After reassembly of the rear brake and drums, after bleeding and adjusting, ... while the rear of vehicle is still off the ground.... I remove the drums again and have a helper slightly depress the brake pedal. I measure/ compare total shoe movement of shoes on both sides to see if comparable.
Good for about 50 k miles unless SIL does a lot of trailer pulling. (Ugh!)