Remember, Tabula Rasa lets him copy the concept of anything he smelts, not just its physical properties. So it's about what the thing fundamentally is or does, not just its appearance and can still use the abilitys like getting big if he smelts a celestial.
Eli is from 2017 before infinity-war and endgame so he doesn't know what comes after Guardians of the Galaxy vol 2
And don't expect him to get to multiversal to fast because that would break the story