Choice between Honda cmx 500 rebel 2021 or the Honda cb500x 2021 (Red) as first bike

It probably depends on how many miles you are going to drive in a day and whether you are going to go only on the highway but also on the road?

Cb500x, versatility is a big win.

cb500x will be more flexible and nimble for a first motorcycle

My thoughts, it depends on how you want to pack:
Rebel soft bags and backpack with sissy bar, and I see more comfort in long distance.
The X you can hook up panner's for hard shell packing, and with all terrain tires you can navigate better on gravel/ off road conditions, down side is them tires are not to good in wet weather. So in the end it would be How You want to travel. And be safe in your adventure.