The pagesize issue does not affect the radeon driver, only the nouveau driver.
Your issue is different, let me guess, you are using the gdm3 login manager?
You should use xfce desktop and lightdm login manager. I
Disable gdm3 by performing the following command.
service gdm3 stop
start lightdm by performing the following command.
service lightdm start
You should have an aqua coloured Debian login screen.
Ensure you select an xfce desktop session
The other desktops like gnome3 and cinnamon, along with gdm3 login rely on having hardware accelerated 3d working properly, which it isn't, we only have 2d accelerated for now.
We have big endian issues with mesa. But other than that, please provide feedback!