As your package depends on cmdstanr, you may consider listing your package on R-multiverse (https://r-multiverse.org). This is a new initiative supported by the R Consortium and backed by R-universe infrastructure.
This should make installation much easier as it will pull cmdstanr directly from R-multiverse, and the other dependencies will still be drawn from CRAN.
As your package depends on cmdstanr, you may consider listing your package on R-multiverse (https://r-multiverse.org). This is a new initiative supported by the R Consortium and backed by R-universe infrastructure.
This should make installation much easier as it will pull cmdstanr directly from R-multiverse, and the other dependencies will still be drawn from CRAN.