As someone who uses R & RStudio on a daily basis for their job, owns an aarch64 Linux device, and has put a fair amount of time into researching this, I can tell you that the RStudio avenue is going to be a dead end for you for now. On face value, the RStudio AUR packages appear to have few dependencies, but the dependencies required are hard-coded into the installation process, (many of which are x86_64-specific binaries), and the AUR packages haven’t gotten around to replacing them with libraries that can be acquired from the repos. Several of the dependencies need to be compiled using Haskell (such as pandoc), which isn’t available in the aarch64 repos yet. The Glasgow Haskell compiler can successfully be compiled from source on aarch64, but I never got around to trying to use it to build the required dependencies.
At this point in time, your best best is to install RKWard & use that. You can configure it to more or less resemble RStudio. But if you go down the route of trying to install RStudio, you’re going to have to have a long road ahead of you.