Wheel keeps going out of alignment and loosening the chain
The short answer is the axle nuts need to be done up tighter than you are doing them up it now. If you can't tighten them more, you need a longer wrench to be able to apply more torque.

There are tension adjustment devices that fit over the axles and in the dropouts that make tensioning the chain and aligning the wheel easier, and help prevent the wheel moving.

According to Park Tool, the nuts should be tightened to about 25Nm; see their video on single speed (or IHG) bikes for how to estimate that torque if you don't have a torque wrench.