I disagree. Trying to generalize a single implementation into an interface is always problematic. And that's exactly what inserting an API between the FE and BE

I disagree. Trying to generalize a single implementation into an interface is always problematic. And that's exactly what inserting an API between the FE and BE is if your only UI is a web page. If, later on, you want a mobile app, then an API can easily be deduced from your current, working UI, instead of trying to imagine what the UIs of your web FE and mobile app have in common.