Installing APK without Google Play Market means you are trying to install application that might be included some viruses (It doesn’t matter if you include virus code or not).
That’s why Android does not allow to install any apk files automatically except Google Play Market.
(Automatically apk install means the Google Play Market app installs the downloaded apk).
Without root permission of Android device, impossible.
(And only one who has the root permission for all android devices is Google)
Of course, if end-users are possible to install through some steps by manual.
At least, they needs to allow ‘unknown source apk’ in the developer settings page, then open the apk file from Download folder.
But I don’t believe all your customers do that.
PWA, progressive web app, allows end-users to kind-of-install small application from browser.
Ionic should work as PWA app.
However PWA app works in sandbox. Only limited features are available.