I had a simlar need (RStudio Server 1.4, Ubuntu 20.04), and was able to get the desired result by adding UMask to the systemd service file.
For my Ubuntu system, I did:
sed -i 's/Type=.*/&\nUMask=0002/' /lib/systemd/system/rstudio-server.service
I think the service files on Centos will be under /usr/lib/systemd, so make sure you're looking at the right file, and you might want to hand edit to be safe, in case the layout isn't the same. I don't have a Centos system handy to check.