Legalize drugs, would end the cartels instantly, destroy big pharma, work towards ending homelessness, remove incentives for coming here illegally, and like you say dont just let it be open season. It was the same way with alcohol prohibition, you only can pull the morals BS with people for so long, those who want alcohol or drugs will get it, cartels and gangs will form to produce them. Much better to have it out in the open, where you actually know what you are paying for. Fentanyl wouldn't be an issue then because nobody fucking wants fentanyl. The ONLY reason fentanyl is even a common thing now is A. It's cheap to produce but mainly B. It's analogues were for the longest time completely legal in most countries, still might be depending on the analogue and the country. There's a reason why if you go to any Lab that tests drugs and posts their findings online you'll see a ton of fentanyl analogues.

By the way, the exact same logic they use to ban drugs gives them precedent to inject you with vaccines you don't want. The precedent of course being whether or not you can choose to consume a substance or not. If the government can prevent you from consuming something they can twist that into making you consume something.