I've noticed Mike Mew talk a lot about diet in his videos, and how the transition to cooked and softer food has left people with weaker facial growth as opposed to eating hard food. Would your jaw muscles develop better if you were to change your diet and eat harder foods daily? If so, what foods would you guys suggest eating?
Bagels are a good start. French baguettes, that sort of thing.
Raw celery is a good suggestion (from one of the above threads).
BTW, any thoughts on whether chewing tough foods or Falim/Mastic gum might worsen existing jaw joint problems?
You should eat fruits and vegetables, the crunchy kinds in particular e.g. carrots, apples, celery. And meat even better e.g. grass fed bison, salmon, grass fed cattle. If you want to compensate from a day where you only ate soft foods, chew regular or mastic gum.
But don't over do it you can get TMJ and if you're switching to a hard food diet, change your diet gradually, so you'll have enough jaw strength to chew those foods mentioned here and above.