Here is one review for your consideration OP. This is my exact experience. I could have written this.
I have had waterpiks for years, and have always notice mold and always thought by running bleach threw the water pik it would solve the problem and cleaning all the parts...after googling mold in water piks, I decided to clean it once again which I do every 2 months, I ran bleach, thru the system, poured bleach on the handle, hit the side of the sink, black mold came out as usual, and then stopped...curious me, I decided why not open the handle and see if the bleach solved the problem, maybe it’s time to buy a new one... I then opened the handle...the black disgusting mold was shocking! And this was after a big clean of the unit...if the water pik company doesn’t come up with a solution after buying one every year for 30 years or more, I won’t be poisoning myself any more...
User's recommendation: Stop using it and realize you don’t know if there’s mold in the unit unless you break apart the handle as I just did...
Product or Service Mentioned: Waterpik Water Flosser
https://waterpik.pissedconsumer.com/mold-in-my-water-pik-202004031851668.html