Unpopular opinion: Eisenberg as Lex could’ve worked...

The problem was the writing.

Yeah if he acted the part differently, the role was written differently and he was directed in a different way then sure he could have pulled of the role, unfortunately none of that is the case and he was a pile of crap in the role.

Funny thing is if they had explained why he acted the way he did I wouldn't have minded too much, like we see in the extended cut he see's Steppenwolf, what if they tied his actions to Darkseid, like he was unknowingly a pawn in Darkseid's plan being influenced by him, bringing out the qualities that drive Luthor to hate Superman and challenge him to their most extreme levels, which you could easily transition him into a more subdued version later on after Darkseid's influence is gone.

This way it makes Steppenwolf's arrival in JL part of a plan, they use Lex to take out Superman because the Kryptonian is the only real threat to the New Gods plans, as it is now it's just luck on their part that Lex happened to want to kill the only guy on the planet who could stand toe to toe with a New God and win just before one of them decides to head to earth for some hijinks.