I've been told very little with the drug dealing system, but in reality every server could always improve.
In terms of cannabis, if it's at all being spawned in (which it should be, considering there's a lot of legal states that have better quality than your average homegrown, and all of that weed IS being smuggled in to prohibited states. In these days, there's illegal grow ops in the legal states that are damn near dispensary grade that are grown for smugglers, when these smugglers pick up it's about 20-100+ pounds at a time. Then these smugglers know a lot of people that sell QPs, HPs and full Ps, and then those people sell QPs (4 ounces) down to about half ounces, of course they'll do smaller but they won't be motivated. Then there's the little dudes that pick up a couple ounces and sell dime bags and higher to try and flip a profit. The process is similar for other drugs (excluding the TOTAL amounts of said drug) but requires more steps as it's probably coming into the country from Mexico.
There's many positions in every part of the drug world, but that's at least how most weed is sold in the midwest, where there are no legal states (except Michigan, but they aren't fully swinging yet. Can't wait to try out what they can grow.). However in an RP sense, there's only a certain amount of people that get access to larger amounts in game, when in reality any new player can take over if they got the cash and trust of their new plug, at least that's how it should be imo.
Edit: I forgot to mention something about your suggestions, fuck me. I'm really digging the idea of different quality drugs and the changes when you use better quality drugs. I'd also like to see some sort of drug task force that could call in the DEA if shit's really bad, but definitely don't have the DEA focusing on the same group forever, it can really ruin a gang (OOC motivation goes down) that can't move anything or get recruits when they know they're constantly watched for weeks on end. I say this mainly for OOC reasons, because it's okay to have crime in a city with limited amounts of gangs when in reality there'd be many more sets for LE to watch.