Hi Michael,
> brms appears to also need additional packages that are failing to install: V8, rstan, shinystan, projpred.
It is indeed frustrating to deal with various dependency issues in R package installation I also experience this kind of problems from time to time. You may try to install those packages (V8, rstan, shinystan, projpred) individually at the R prompt, and then trace back to the root of the problem. This could be tedious, but it may work.
Gang