Risk of driving when smoking weed is not a good example, because it is illegal to drive while high - much in the same way that it’s illegal to run a red light or illegal to discharge a firearm into the air within city limits - the exact same arguments apply, where the victim is those other bystander who is exposed to risk. Taking two otherwise legal things and combining them makes it a risk to others, and illegal. Same as drink driving, either drinking or driving separately is not considered a risk.

The health insurance thing is a better argument (especially if you’re in a country with single payer or otherwise taxpayer funded healthcare). The threshold here is a little more dicy and somewhat subjective, but the core argument is good. Cigarettes are legal, and far more carcinogenic , with a far higher risk of respiratory illness, than cannabis smoke (assuming we’re not talking about THC gummies or whatever where the medical costs associated are lower), so if this line is somewhere where things like cigarettes, diesel combustion engines, alcohol, coal fires power plants etc are legal, it wouldn’t make sense to make low impact drugs like THC illegal.

So to your first point, we, as a society must have some threshold where we accept some risk, otherwise pretty well existing would be illegal (what if you contract a contagious disease and kill someone?). The main argument here is it should be consistently applied. If the cost in respiratory illness caused by sulfides in coal fires powerplants has associated medical cost of exposed people orders of magnitude higher than the total sum of cost associated with individuals using a particular drug, reason would dictate that if the impact of sulfides is considered acceptable that the far lower impact of that drug is also acceptable. Both of these examples carry negligible risks compared to the more deliberate and dangerous actions like running red lights or firing guns in populated areas, so these could still be illegal with consistent reasoning.