I have a single speed gravity bike with carbon fork and caliper brakes similar to this one: Save up to 60% off new Disc Brake Flatbar Road Bikes - Gravity Avenue FXD | Save up to 60% off new road bikes

It currently has 700x25 tires on it and has 120mm long thru axle (I don't know how to measure diameter but the middle of the hub was about 18mm)

I have broken 2 back wheels and 1 front one in the past 5 years so I'm hoping to get a more durable one in $100 price range if anyone can suggest something. Thanks

I am not sure if this additional info is required but it is a track bike with flip flop hub

What's breaking? Rims? Spokes? The hub? It's hard to point you in the right direction when we don't know either what is failing or indeed what you are riding. (I went to your link and all I see is straight bar road bikes that cannot be (easily) converted to fix gear. Disc wheels. You are breaking wheels, esp rear wheels but flip-flop fix gear wheels aren't compatible with disc brakes so I am at a loss to help you. (I've been commuting on fix gears for 40 years and have two with flip-flop hubs so I'd like to be able to help.)

Ben