ሰላም! One option is to use IBus with the Amharic - sera layout. Try this:
System > Prererences > Keyboard Input Methods. If this is the first time you use it, it will likely ask whether the ibus system should be started (asked twice for me), you should do that.
The "IBus Preferences" dialog should show up (if it doesn't, try the menu option once more). From the dialog, choose
Input Method tab, and select
Amharic - sera (m17n) and click on
Add to make it available.
Try if it works in your applications, by switching between normal and IBus with Ctrl + Space, for example in LibreOffice: Ctrl + Space, "selam", Space, Ctrl + Space, "hello" should input first the correct fidäl and then English (assuming that is your default keyboard layout).