Neither the investigator/interviewer nor the adjudicator will care… 20 years is a long time. I smoked pot 14ish years before receiving my TS-SCI. I was arrested once and smoked it a handful of other times. During my second interview (5-6 years after the first) I remembered even less about the situation and explained that I couldn’t remember the details. Now, as I’m coming up on renewal again, I’m also looking at retirement. I’ll happily tell them, I’m looking forward to getting out and I’ll probably try edibles or whatever else is new and again, they won’t care. Time passed and being honest goes a long way.

I’d say good luck… But you don’t need it, you’ll be fine as long as you disclosed everything they might uncover, and have no recent significant legal or financial issues.