There you have the first problem: RSTUDIO_WHICH_R must point to the R binary, not the path containing it, so you probably should set something like RSTUDIO_WHICH_R=/opt/R/4.1.2/bin/R in your profile.
But since it still points to /usr/bin/R, something else must be wrong: Depending on the type of shell you are using on linux, your RSTUDIO_WHICH_R variable definition could be overwritten by other files sourced at shell startup, e.g. for bash ~/.bash_profile or ~/.bashrc, for tcsh ~/.cshrc and so on (you can find the type of your shell via echo $SHELL).
You also could have added the RSTUDIO_WHICH_R definition to the system-wide config files in /etc. Try running grep RSTUDIO_WHICH_R /etc to see if this finds something.