I gave Braunvieh a vote, just so they wouldn't be left out... I really like what they bring to the table as a terminal cross, but still look at them as a maternal cross, as well.
I'm guessing these RAxChar cows are 'yellow' or white?
I'm not exactly sure what a Braunvieh over that combo would give you, with regard to color, but you can bet that there'd be a bunch of brindles in the offing, though they might be really dilute.
The Braunvieh 'wild color', when combined with 'red' often gives you a brindle (see photo).
While Fleckvieh are Simmentals, they are not necessarily 'equivalent'. I would anticipate heavier muscling and probably more 'leather' with a Fleckvieh sire than a typical Simmental, but epd profiles of a given sire may need to be considered as they meet your needs.
We had significant Fleckvieh influence in our herd from ~1989 going forward, and the last couple of years, were using a fullbood Fleckvieh AI sire over most of our better cows.