Yeah, well, unfortunately....when you go down this road of estimating and validating harmful effects, you wind up exactly where we are now. Except that we'd like to argue that certain drugs have been WRONGLY EVALUATED to have extremely harmful effects, so harmful that they should be criminalized.
I believe that one by one, those sanctions can be lifted. Specific drugs, whose effects are well understood, one at a time. That would solve 80-90 of the problem which might be as good as could be done.
But there couldn't ever be a blanket legalization of feel-goods because in many cases their harmful effects might not be known or fully evaluated. Plus new ones will arise and often.
OK you've lost me now, what do you mean by "go down this road of estimating and validating harmful effects"? Are you suggesting this is how the state decides what drugs should be illegal? Because that's absolutely not how it works, most drug laws are due to political reasons and not based on evidence.
I think that's what you mean by "wrongly evaluated"? That some of the safest drugs are illegal, and some of the most dangerous are legal? That's certainly true, stuff like weed and psychedelic mushrooms haven't killed anyone, but things like Fentanyl are extremely dangerous in the wrong hands.
Your idea of legalizing drugs one by one based on their harm profile isn't a terrible idea, far from it. However it still doesn't address the fact that users of drugs that are deemed "too dangerous" will still be criminalized, denying them help, and also that the production of said drugs would stay underground and be unregulated, causing more death and injury to users.
Your last sentence demonstrates a misunderstanding of how the drug market works, people are not going to experiment with new untested drugs if there is already a legal option. A good example of this is the rise in "legal high" type drugs, which people only use because the best drugs are illegal. If MDMA, Cannabis, LSD, mushrooms etc. were legal, few people would experiment with lesser known drugs with barely any human experimentation.
As someone with personal experience of this legal high/research chemical market, I can tell you that it does self-regulate itself to a degree. And also that most deaths due to these newer drugs are due to unprofessional vendors mislabelling drugs, or naive users not being careful enough with dosages. A good example of this would be the incidents involving Bromo-Dragonfly or 25i-NBOMe (worth googling). The vast majority of deaths like these would have been avoided if LSD and mushrooms were legal.
The vast majority of deaths due to opiates like Heroin are due to impurities and incorrect dosages. By keeping it illegal, these deaths are not being addressed and will continue. From a harm-reduction perspective, it makes sense to legalise them and allow users to access pure product of a known dosage from a controlled laboratory. Just because it's legal, it doesn't mean that every schoolkid is going to start mainlining Heroin.
A proper education programme would address things like this, show schoolkids the real evidence of how dangerous certain drugs are, explain the risks of different types of drugs. Tell them that ALL drugs can be dangerous, but if you really want to take them, stick to the safer ones and be careful how you take them.