First thing I need to ask is are they neutered? Their life expectancy will jump to the 10-15 years ranged right away once get it done. Otherwise you'd be lucky if they get to 5, in any rural setting.
You could get a few years if you hired someone to come to shoot the feral dogs once a week. You couldn't do it yourself as you'd need a real gun. Poison can't do, as you have cats. Even though cats are way smarter than effing dogs.
Anyhow, neuter...