Hello,
Have a Pi4. Xscreensaver is the only screensaver I can find for Raspbian. Is there any others? Problems I've had with it: Won't open, window flashes then closes. Fix is to go Menu / Preferences / <Right click Properties>Screensaver. Click Desktop Entry tab and check Execute in Terminal, then OK button.
Then when you open it you will see the first error message which can be solved by Googling the CLI error and then downloading some extra packages. When done, you can go back to Properties and uncheck "Execute in Terminal Window".
Xsceensaver though will do as you set in the Screensaver Preferences, except after a certain time the screensaver will stop and the login box in blazing white appears.
I realise that "By default, the Raspberry Pi Desktop does not have any easy-to-use screensaver software installed" ... but why not? Could the devs answer why such a simple app isn't included?