Okay, I remember seeing a documentary where people were doing lines of methampheatmine. And typically people would do 2 to 3 lines and be up all night. I would consider a line a "small amount".
If I could go to a regulated dispensary and do a line of methamphetamine I think I'd try it. I don't think it would be neurotoxic, but I'd be very interested to know data that supports that, obviously! If you have, please show.
The doses you are describing I think fall outside responsible recreational use and into other categories - that sound more like poisoning / overdose.
A problem of criminalization is that society seems to tend to bucket all illegal drugs into a single category of usage misrepresented by the worst doses. It is possible for people to use these substances responsibly. And with assistance on purity, and regulated access, I believe the chances are high.
The regime I am proposing should even make it easier to deal with the % of people who bucket into overdoese/poison territory.
I'm not sure how much 1 line contains, but to me it seems as if it would be 50mg?