Oh boy. Buckle in, this is going to be a long one. This question crops up periodically on a number of other social media sites, and below is my answer to previous inquiries:

I think for some people, 4th Edition was exactly the sort of game they were interested in. But conceptually it was a departure from the aspects of 2nd edition, and even 3rd edition, which I’d come to enjoy.

Folks have said it was too “videogamey” and certainly it did seem to use some team-videogame concepts, though I wouldn’t say that was exactly what rubbed me the wrong way.

I can think of three options, none are perfect, but each could be tinkered with to get what you want:

In the Light of a Ghost Star – This is much further in the future than you intend. But it has incredibly simple rules which are easy to customize. Also I made a psionics supplement for it here. I had pretty good success running it.

Mutants & Machine-guns - A loose, fairly tongue-in-cheek system. Free.

Mutant Future - Basically a cleaned up clone of early edition Gamma World. Less light than other games mentioned.