Analysis looks correct, although I think herd immunity wont happen until 2021.
It's quite possible that by 2021, those infected in early 2020 will no longer have immunity. While we are hoping for the best, it seems unlikely CV19 confers permanent immunity. The question is how long will immunity last? 6 months, 12 months, 18 months, 2 years? Still, even if 25%-50% of the population at any one time is immune, that slows down the spread and helps reduce the load on our hospitals until (if) a vaccine is developed.