After days of trying-out-everything-else-agony this finally worked!

While having the install screen (with the greyed out button) of an apk shown (through a file explorer ofc), press the settings button on your remote twice.

No trolling, I was flabbergasted too, but this was working for me.

Amazing 'rhodzy' posted this here https://rzy.co/2018/04/25/install-button-greyed-out-philips-smart-tv/