It is very rare in my experience to have more than 3 or 4 cats living in one house without inappropriate urination issues. You have gotten good pointers so far and the only thing I can think to add is that I would bring some litter boxes inside for more easy access and the rule of thumb is 1 litter box for every cat +1.

Good luck believe me I know how distressing this can be which is why I only do hardwood or laminate floors so I can throw cheap oriental type rugs down and toss them when they aren't able to be cleaned.