There are estimated to be about 100-150 copies of Action #1 left in existence in all condition grades. And the one that sold for $3.2 million was the highest graded copy known. There’s one other that is graded the same, but that copy isn’t as nice per most who’ve seen both.
So it’s a unique item. Just like the Lotus painting.
You’re right that any Original Art from Action #1, featuring Superman, would be extremely valuable. But it’s hard to say whether it would be worth more than that action #1. Interior art, maybe not. The Cover, almost certainly yes. But the art from that book probably no longer exists.
The most valuable known comic art in existence is probably some of the interior pages to Amazing Fantasy 15 featuring key moments, or the cover to Hulk 181 (perhaps). Others can probably suggest additional candidates, based on their better knowledge of what’s out there.
But all it takes is one person being willing to pay the price asked for something. I highly doubt tat the owner of the Black Lotus painting would turn down $2.5 million for it, if it was a serious offer.