What about sodium triphosphate in drinking water?
It appears to be safe, provided that you are putting it into one of the delivery systems that it’s made for which I found through the link you provided.
Here is one of the delivery systems:
https://www.pineco.com/eng/products/dosing/polyphosphate-dosers/spillo-polyphosphates-doser-dm
Per the specs, it delivers 3ppm, which means that you’re only introducing 1ml per 333L of water, which is approximately what the average person uses per day.
Contributing to phosphate load shouldn’t be a concern as someone (with “normal” diet &amp;renal function) has a concentration of 10-300ppm of phosphorus in their urine!
This company also sells water filters for regionally specific water issues like high arsenic, and water softeners with different ion exchange resins based on the ions present in the water. They appear to have a solid understanding of drinking water chemistry, but it’s your domestic water, not mine, so caveat emptor