You don’t need xscreensaver with Plasma Wayland. The lock screen can be used as a screensaver.
First, open System Settings → Screen Locking. Set the amount of time before the screen will lock (i.e. display the screensaver), and whether your password is required for unlocking. You can also set a delay where the password is not required for a specific time after locking.
Then, click on the Configure Appearance button at top right of the lock screen settings & select what you want to display while the screen is locked.
Note also that the lock screen uses the same plugins as your desktop wallpapers. To add more plugins you can right click on your desktop, select Desktop and Wallpaper, and then click the Get New Plugins button.