Illegal drugs creates an innovative, unregulatable black market and ultimately promotes corruption, a force that grows exponentially.
De-regulation has very little to do with the drug users themselves or their harm to society, and everything to do with destroying the economy that those drug users support. The cartels are beyond regulation, but the Sacklers could be put in court. That's a pretty big difference.
If fentanyl is a life sentence in the way you say it is, the cost to society is less if we (ethically) let people kill themselves with regulated fentanyl usage in supervised zones rather than unethically in the streets with drugs of questionable purity purchased through questionable money.
If those people are desperate enough to commit crimes to pay for their mental escapism, then giving them their fentanyl without the crime, while ethically complicated, could have better outcomes for society.
Having pro America institutions (health care) interfacing with people is going to have better outcomes than anti-American (drug cartels and foreign powers) institutions interfacing with people.
Have you ever seen a needle on your sidewalk, in the park your kids play in? Have you ever seen people use dirty needles in front of you? Prohibition doesn't work in America. You can't just make it illegal to leave a needle in a park and expect needles to disappear. Making it easier to do a less bad thing is a different kind of policy than paying more for enforcement (in the form of more taxes in order to pay for more police officers).
Having freedom means being able to make the wrong choice. Keeping freedom is about making responsible choices. If you believe in freedom, then you have to figure out how to help people make responsible choices and have a plan for people making the wrong one.
Monopolizing the supply chain of drugs allows you to dis-empower and destroy cartels as well as offers a "hook" (in the programming sense) to be able to influence the path of those affected.
So you're right that "tax and regulate" makes sense for weed and not fentanyl, but I also don't think you see that there are other forces at work in terms of power. If america is the supplier, america has power. If cartels are the supplier, the cartels have power. If China is the Supplier, then China has power.
If China has that power they can use it to foment misery. If America has that power, they can use it to mitigate the problem.