Except people start reselling legal drugs to the jurisdiction where they remain illegal... For this to work out as intended it would have to be planet-wide.

Well, the criminal networks are planet-wide. The war on drugs is planet-wide. Thus, the legalisation of drugs can also be planet-wide.

a few things:

1) illegal drugs are still illegal, just like now.

2) the quality of the illegal drugs rise significantly if sourced from reliable, regulated manufactures

3) the "blood" cost of production drops precipitously

4) the end user cost drops as well.

Now without support, those addicted will still flounder. However they won't need to do as much crime to fund it (much better for society, worse for the individual)

Sounds like a problem for that other jurisdiction.

Ideally, it could also be great source of tax income, which could be further used to help people get rid of drugs.

Another thing. The problem is addiction, so why not more directly target that? Much like with added sugar. We should not put people for years in jail then call them a permanent risk to kids no matter where they actually are after, simply for possessing sugar, but it should not be so cheap. So make the addictive stuff cost a lot of bank by heavy taxation (say 300%, of which 100% then goes to funding rehab and services), make mental health services very easily available by comparison, and if coercive interventions are to be added, make them in a form of actual restraint more than punishment, i.e. the intent is not to cause suffering so much as to physically keep the drug away (e.g. putting people around them and/or working with their surrounding community to literally keep the drugs away from them [incl. by snatching it from their hand if it comes to that] instead of putting them in a dungeon where they are fed food that is intentionally chosen to be dangerous or with rapists that could very well lead to a fatal outcome).