Advice on what to buy for someone who snaps rear axles often
You almost certainly have a 7 or 8 speed freewheel hub. They can do this, especially 8. Google those terms along with broken axles.
8 speed freewheel hubs had some years of prominence circa 2000-2001 and then were rejected by the industry because of these problems. They have now made a return due to manufacturers answering the pressure to fit so many other name features into a low price point. They're cynical trash.
If the above surmising is true, get a compatible cassette hub wheel and a cassette, plus a new chain.