Sure, there are drugs that are worse than others, and seeking to redirect use where possible would be good. Especially when dealing with addicts.
Some people do seek out opiates especially because of its specific effects, and I don't think you can entirely prevent that, though you may be able to reduce it with better mental health support, but even then you could also redirect use to safer options, and also segmenting out those seeking opiates from those seeking party drugs etc. so that you don't create pathways between them might well be beneficial.
To stick with my heroin example, I don't know where specifically heroin sits relative to opiates that would actually satisfy existing addicts and that's part of the challenge, but heroin itself is also nowhere near the worst opiate, and one of the negative effects of the current way we're handling this is that we've created a pathway towards more dangerous ones.
E.g. a frequent pathway to heroin use today is from oxycodone, to oxycodone on the black market after you lose your prescription, to heroin. But then it gets worse: Dealers cutting heroin with fentanyl or even carfentanyl.
Getting users nudged gently or not so gently towards the safest options that will satisfy them would in itself likely help a lot, and of course, a black market dealer has no incentive to do that unless there's a very severe difference in how they're treated by law enforcement.
Perhaps a variant of what you described would be a good first step in many places not ready for more decriminalization: Much more aggressively segmented drug sentencing by the harm of the worst drug a dealer can be connected to, and perhaps even explicitly punish "crossover" between drugs with differ harm profiles to reduce the pathways between them.
But that means being honest about the abuse potential, and not politicizing it, and that is in itself an uphill battle in many places.