Call me a skeptic, but in this market I will keep using Toyota fluid and spheres. Tested and proven seal compatibility. In some other markets retrofits might be significantly cheaper and justified. I have seen refill valves working in Siberia years ago. The guys were testing Argon vs Nitrogen pro and contra. For our market, I can afford $850 every 17 years (my 2002 recently) genuine Toyota from Japan. The fluid is around $15 per litre here, if you add shipping from Europe it will be even more with a questionable long term material compatibility.
If one reads in other languages, you can find quite a bit on diaphragm and fluid material science evolution by Citroen