I’ve pushed an updated version of the manifest here: https://github.com/Alexander-Wilms/flathub/tree/rstudio
The development version of RStudio now uses Qt WebEngine, which is already contained in the Sdk, that might fix your memory-related issue. If that doesn’t help, I can upload the .flatpak bundle somewhere.
EDIT:
Flatpak: https://drive.google.com/open?id=1zbS8qOxikEnvS_RfaF22kHsnAYSSRhJO
sha256: fa536aadd981b579bffc9ad33dd48413cd1dcad5496137368aaa419bf56d2e19