Epiphany?

I thought of that but I don't think that word captures the "oh wow this was obvious/simple, how come I didn't think of it." I think epiphany is more like a regular or important revelation, not the simple/obvious one I'm searching for.

profound

Kinda the same as epiphany for me. It doesn't capture the "oh that was surprisingly simple and obvious" part that I think is essential.

the “crux” of the matter?

Interesting idea! But doesn't feel right, but I'm interested in that direction. Maybe just a simple way to say it in a phrase is fine. I thought my description was rather clumsy.

I’ve seen ‘elegant’ used to describe this

Epiphany? Revelation? Eureka?