Could you hook it up to a proximity sensor that just toggles the visibility of the icons if the character is, say, ~2 feet or less away from the interactable? Or would that still make it too easy?
One solution that I've seen some more 'traditional' point-n-click games use is to make a little periodic twinkle, just a little subtle one, on stuff that can be interacted with. I'm not sure that that solution would work as well for 5DD though, if only because of the art style. But perhaps something along those lines could be workable, a tiny animation, or even a fly circling around an item you can pick up, idk, it's your game and I'm just spitballing here lol.