@muratmaga is more knowledgeable on this idea of going between Python and R options, but I think you will have to rely on installing python packages that call R packages where you’re still using a python 3 interactor instead of an R interactor.
Starting from this post linked below in a really old thread you can read some about why development of R within Slicer is likely not going to be implemented. It’s about most R packages having a GPL license instead of LGPL and Slicer is not wanting to distribute any GPL code in Slicer packages or extensions.