How would Jenson Button fare in today's Formula 1?

It very much depends on what kind or car you give him…

I think his smooth driving style would have suited the ground-effect cars. I can't imagine he'd be competing for titles again though.

He's obviously not on the level of Verstappen, but he could his own against Norris, Leclerc, Russell etc

I completely agree! Not quite at the top but amongst the stronger drivers!