It’s perfectly possible to install e/os directly on Android15.
The installation script deletes all data on the phone. This is a clean installation.
It is necessary to install the latest version of Android SDK Platform-tools beforehand:
https://doc.e.foundation/pages/install-adb
Then follow the e/os documentation to flash the phone via command line:
https://doc.e.foundation/devices/tetris/install
Best regard