As you've discovered, the down side of United's program is that saver inventory can get extremely scarce once a regional jet leg is introduced into the route.

The down side of Southwest is that they can get surprisingly expensive once the Wanna Get Away fare buckets sell out for a flight, and since it's revenue-based redemption...

The Southwest + rental car as primary makes sense, but it's easy to build and maintain a stash of United miles through non-flight means )credit card, shopping portals, shopping app, etc.) and there could be times ahead where the 50K United miles + $85 non-elite close in booking fee is a screaming deal if you need to visit someone in the hospital in Illinois 12 hours from now.