I have a Diamondback Outlook Omni 181 CL, bought it second hand for $40, and I'm not sure of the original parts. Recently I have been having trouble with the rear axle, the one it came with bent so I replaced it, and then next two bent. I don't do anything extreme, I ride ~6 miles a day, mostly on sidewalks. I'm tired of replacing the axle, I'm tempted to switch to a solid axle, as I currently have a hollow one for a quick release but I don't use that feature often. I don't know if the axles I have gotten are just bad or if there is some damage to the wheel that is causing the axle to bend, but I don't see any damage to the wheel.
Broken axles are an inherent feature of the 'freewheel' hub design. They happen because the load from wheel bearings is close to center, where it has more leverage than closer to axle ends. Newer 'freehub' cassette designs solve the problem with either having bearings inside the cassette close to axle end or with oversized axle.
The best option would be replacing the rear hub with a freehub type. The easiest way of doing this is replacing the entire rear wheel. If this is not an option, an original axle from known brand should be expected to last longer than random junk from Amazon or Aliexpress or at least have a warranty. A solid axle does not help, because the reason for breaking is tension from bending the axle. The tension is greatest at the surface of the axle, so solid interior does not help but the compression from quick release skewer slightly reduces it.
The best option is to acquire a new rear wheel that is equipped with a more modern freehub that accepts cassette sprocket clusters. The freehub puts the bearings more outboard and that helps prevent bent axles because there now isn't such a long stretch of axle unsupported like is found in the right side of freewheel systems.
Your bike is likely a 2002 Diamondback Outlook. Possibly '03. In any case, new rear wheels with similar specs to your current--albeit with a freehub--are reasonably priced and the better deals include skewers and sometimes a cassette as well. Seven speed cassettes also are very reasonably priced. Here's what you presumably have for a rear wheel and here is the same model wheel only with a cassette hub. You can still use a seven speed cassette on this 8/9 speed hub with the addition of a spacer that slides on prior to the cassette. Please keep in mind you're not limited to this model (or that price point) of rear wheel. These links are examples illustrating the subject.
Many areas now have "bicycle coops" where good used parts can be obtained for very reduced prices. Same situation exists online in the form of e-stores and forums within sites like Facebook, so these are additional options to get a better wheel without spending more than you bought the bike for. Hope this helps.