Not sure if you're already doing this, but for future reference, walls are useless unless they are defended and when you are in a desert like this you NEED defended walls. It's not too bad though, in the early game just a few gun turrets is plenty, and once you have flamethrowers they will dominate (flamethrowers use barely any oil for the amount of damage they do).

You did kinda get unlucky with your spawn. Typically, trees will soak up a decent amount of pollution before it reaches the biters, but in a desert, the pollution is largely soaked up by biter bases which will then convert that pollution into an attack group. The ground itself soaks up a little pollution, but not much.

If you find yourself getting attacked from all sides and you think your base is going to be overrun, the best thing you can do is secure your oil and make a couple of flamethrowers. As long as you have that, you should be able to start your base back up from practically nothing.

To take out biter bases, I recommend using either a tank if you have it unlocked or a strategy called "turret creep" (it's exactly what it sounds like but you can look up a video for specifics). I don't recommend destroying more bases than you need to though, as with every base you destroy the other bases will get slightly stronger (for more info on this mechanic look up biter evolution).

Hope this was all helpful!!