Your argument is intriguing but you haven’t explained why this would help keep people from dying of fentanyl overdoses?
My proposal would sell pharma-grade drugs of known quality/dose in grocery stores at affordable prices. The illegal market would die overnight.
So why not treat other drugs the exact same way? There is no way to be logically consistent about this UNLESS you think that alcohol should also be illegal.
Rather than some off white powder that contains an unknown amount fentanyl.