To me, decriminalization of personal use means that I can smoke weed, or maybe even meth or shoot heroin, and not be punished for it. This would minimize the harms that come directly from the government enforcing drug policy, for something that typically is not going to hurt anyone but myself.

Whereas legalizing it would mean that I'm free to manufacture meth and heroin and market it, and sell it to others; I would still have an incentive to use some drugs' inherently and biologically addictive nature to profit myself at the expense of others.

I specifically say "inherently and biologically" because many things are addictive to a degree - alcohol, tobacco, video games, in-game purchases, gambling - and any of them can ruin some person's life. But they don't appear to be as universally addictive as, say, heroin or meth, which can have terrible repercussions even if you stop using them. It's a difference of degree, but it's a significant one, and I want to head off at the pass any argument that selling meth to my neighbors is no different than selling them sugary soda or cookie-clicker clones.

I don't think you can kill the black markets unless you allow anyone [that can meet the regulatory burden] to manufacture and distribute.

And you can't tax them too highly, either, or those black markets spring right back into existence. See moonshining or cigarette smuggling for examples. A 25% excise tax at the point of sale to the consumer is probably the highest you can go. Taxation is, after all, one of the ways in which prohibition was backdoored into existence, and the amount of the tax is the maximum possible profit margin for someone whose sole business is evading the tax.

That leaves open the question of how you keep the most dangerous drugs--generally the opioids--from ruining people. That's really a moot point until the end of prohibition seems imminent. If certain people won't consider any other option, it's useless to throw one out there for discussion.