I didn't tried this on R2S specifically, but if it works inside systemd-nspawn - then it should work on headless SBC too.
1. Install Mate or XFCE via tasksel
2. Install gdm3 and tigervnc-standalone-server
3. Create these two files (don't forget to enable socket) and configure gdm3.
This is of course not only option - you can find other ways to get TigerVNC or xrdp works on headless system. I just told you what works for me, that it.
