Your tires are fine, and upgrading to 12" wheels was a good idea. That slows the RPMs of the wheel bearings down so they last longer.
I would not flip the axle. Generally axles are not perfectly straight. They have a slight amount of positive camber built in to promote side-to-side stability. If you flip the axle over it will have negative camber and the trailer will be more prone to darting from side-to-side behind you.
Tow the trailer level or slightly nose down. Make sure you have at least 10-15% of the total trailer weight on the tongue. Watch it carefully to see that it tows straight behind you with no constant darting from side-to-side.