Been around the scene in SA for 15 odd years and have friends/friends' kids playing in the US in high school and college
I'd say High school varsity is about the same as u21 div 1 here on average, obviously super elite schools in america are much above that (IMG, Montverde etc) and pretty hard to gauge what they would be, the above average high school varsity team is probably about the same as 18s div 1 here and the average high school varsity team would be around 18.2s here
the low-mid D3 NCAA would be like the top 8 in SDL1 mens (just below NBL1 reserves) which is equivalent to Big V Div 1 in terms of competition tier
mid-high D3s and low-mid D2s would be like stronger NBL1 reserves teams/Big V champs and mid-high D2s would be about the same as NBL1 teams with no imports
No SA NBL1 team can compete with even average level D1 colleges, thats just the harsh reality
Just my views from watching a lot of ball, no real scientific metrics