You can register a hook (custom function) that is called when tidyverse is attached. For instance, add the following to your ~/.Rprofile file:
setHook(packageEvent("tidyverse", event = "attach"), function(...) {
library("stringr")
library("lubridate")
})
This avoids preloading any packages when you start R while at the same time causes those extra packages to be attached whenever tidyverse is attached.
PS. If you use startup::startup() in ~/.Rprofile, you can put the above in a standalone file e.g. ~/.Rprofile.d/package=tidyverse.R, which is then only process if and only if tidyverse is installed.