Welcome,
First, I always admire care givers having been a recipient several times. Your Dad is fortunate and pat yourself on the back. Your help is doubly tough with dementia.
Any answer might include just how bad the dementia has gotten. From what you have described with "limited" control of his urination, your plan is probably a good one. The toxicity while real, is probably not quite as intense as you envisage. Washing thoroughly of hands and cleaning the toilet bowl are important. At the same time when I asked my URO about the BCG his answer was "If it were that dangerous, I wouldn't allow my staff to handle it". Clorox/bleaching the bowl is still a good idea.
Edit ad: Should you need extra help, beyond normal channels via insurance/day care etc, some of the hospice care groups do wonderful palliative care. This is not "end of life" but, simply helping patients be more comfortable in a home setting.
