Brake pedal is really stiff

My first thought reading your saga was a common hard pedal problem caused by stuck wheel cylinders. I agree with 67Dart273 that there is still an obstruction in the system somewhere now that all wheel cylinders have been replaced. If the brake pedal won’t depress when connected to system something is not moving fluid or pistons.

First place to look for fluid flow problems are the soft lines which if of fairly recent manufacture will that date on them. If no date or ten years old or more replace them, they are not safe. These lines may look nice on the outside, but can be mush on the inside or have chunks of loose lining acting like a foot valve preventing flow a result of age and contaminated brake fluid. And, as suggested previously, check distribution block for crud as well. These are the two remaining components you have not addressed.
Generally poor stopping performance indicates front brakes are not working very well as they do most of the work.

Diagnostic department:
Air in hydraulic system will compress when brake pedal is depressed giving a mushy longer pedal travel which translates to less force applied to brake shoes or pads resulting in long stopping distances. Fluid will not compress unless heated to boiling point.

Parts not moving such pistons in cylinders or calipers, improper fitting shoes or friction compound, and blockage of fluid flow will cause hard pedal.

Drums and rotors turned beyond design thickness cause a low pedal because the friction material has to travel too far to make contact with drum or disk. This will result in unsafe slow application of brakes and longer stopping distances.