Producers following the laws can not compete with illegal producers on price. So even if you make consumption legal, the illegal networks will remain and become even bigger as the market expands.
In Germany, you always had a lot of "deutsche Hecke" (cannabis grown in Germany) on the market. Lots of small operations. While still illegal, it isn't unethical and certainly doesn't involve slaves...
While more or less impossible to have legal (non-medical) weed right this moment, I very much doubt cannabis grown outside Germany will be attractive much longer. Because it's ethically questionable, because it's been in someone's butt, because it's often cut with dangerous substances, because you can't trust its quality and because you don't want to get involved with street pushers. Technically, you will have a "black-market", but it won't be the ultra criminal gangsters running the show. It's gonna be a friend-of-a-friend who's growing more than they are allowed to, selling off the extra.
There will be a questionable market for "doctors" writing prescriptions for medical weed now. A situation you find in many countries already. That's just the parasitic business you get with still restrictive legislation. Many people will go this route. Not because it's cheaper, but because getting weed this way is trusted, convenient and reliable. Medical weed costs about the same as high quality weed on the street before legalization. Prices will go down, margins will be smaller.
While slave labour is cheap, at scale a combine harvester is even cheaper.
But it's not just the raw production cost that matters. If you add a 200% levy, then ofcourse you risk tipping the edge back over to the black market.