Download this: https://apkcombo.com/screensaver-shortcut/com.computerkim.screensavershortcut/

Sideload it and install it. Then use Button Mapper to map this app to the remote button event you want like long pressing the Home button.

Install Aerial Dream https://play.google.com/store/apps/details?id=com.codingbuffalo.aerialdream

Then set the button to launch it.

Use Flauncher has your launcher and then if u press back it starts screensaver! No need to remap buttons!