I can't speak directly to the Brightline train, though in general I like train travel.
But I wanted to point out an option that you may or may not have considered... instead of a rental with return at MCO, have you considered a rental with drop-off at the WDW Care Care Center? It would eliminate the need for the extra shuttle (Sunshine Flyer) from MCO because you could go directly to WDW, drop off luggage (and maybe people) at the resort, then drop-off the car at CCC with an internal (free) shuttle back to your resort or a park.