Welcome to the RTA Forum!

I think you are on the right track. Your first day, you should try to get a bit farther than Phoenix. I'd shoot for at least Tucson, and possibly even onto Benson AZ. Your 2nd night, Fort Stockton TX would make for a good stopping point. Those would give you 3 pretty even spaced days of 500 miles each, which will take you 8-10 hours a day depending upon how often you stop, etc.

Note, that I-10 does not go to Juarez Mexico, it stays in El Paso, completely on the US side of the border. Obviously with a UHaul, you can't cross the border, but Juarez is one city you certainly don't want to visit (El Paso is completely fine though).

Otherwise, don't worry too much. A UHaul truck is really pretty easy to get used to, it drives like a big van, but there's not that much more too it.