Derp I just remembered the existence of Atom Zombie Smasher. It's kind of an RTS! There's not really any base building but otherwise it's very RTS-y.
I'm fine with no base-building. It seems fun, I'll look at it.
Do so; I got it when it was in the HIB and regret nothing. Although it is honestly more of a TD game than RTS, as snipers are restricted to moving around the rooftop where you placed them, arty is immobile once placed, infantry can only move on streets and parks, and everything else apart from the evac heli falls under static defenses (mines, barriers, etc.). Still very fun though, and the concept is well executed.
The problem with atom zombie smasher is that the default mode the game randomly picks your units for you - getting something like barricades/landmines/dynamite/bait 3 times in a row is EASILY enough to lose you the whole campaign no matter what (even getting a crappy combination like that once at the wrong time can end you). That's just bad, because there's absolutely nothing you can do to stop it or influence it.
On the other hand, if you use the option to pick your own teams, the game is really easy. High ranked artillery can almost win you the game on it's own, high rank soldiers charge around the map with an 80% - 100% hitrate on zombies mowing them down left and right, etc.
There's no real in between, either you let the computer choose and the campaign is as much (or more) about pure dumb luck as it is skill, or you pick and the game is just easy.
(I guess you could select the picker option and then pick bad units, but that's unsatisfying and a game forcing the player to handicap themselves is poorly designed imo)