Just out of curiosity, why are you using such an old R version? You can compile the latest R version on ARM64 as well
Actually, 3.6.3 is what we are upgrading to. You don't want to know how old our current production version of R is. We are working our way this year to R 4, but because there are bigger breaking changes in R 4, we decided that it would be prudent to go slow so that we can trace errors.