Spokes snapping / Rebuild or new wheel.

I have a cheap adventure bike, just over a couple of years old.

After about a year the back wheel spokes started breaking, so I rebuilt
it in January with cheap 10 GBP spokes off ebay. Now after only 5000km
I've had a couple of spokes go again, from fatigue.

I'm surprised the spokes would go so quickly. Is it likely that buying
bargain basement spokes was a dumb thing to do and I should have bought
more expensive double butted spokes. Or is it possible there is
something wrong with the rim or hub.

So now I'm thinking of rebuilding again with better spokes and a new hub
(needed anyway). Would I be better buying a new wheel? This is my first
disk brake bike, I assume the rims are effectively immortal.