You can use BA or Qantas miles only on certain AA flights that are priced low in points on AA.com. That's always been the problem to some degree, but with AA's new dynamic point pricing, it's probably a problem more often now. Before AA changed to dynamic pricing, when it went by an award chart, you could only book with BA or Qantas when AA had the flight at the SAAver point level, not at the AAnytime point level.

So if your dates are fixed, you probably can't use another airline's miles unless you gamble that the AA.com rate will drop at the last minute, and maybe then it'll become available on BA like that May 1 example above. But that may be less likely for summer dates than for current spring (but past spring break) dates, like the May 1 example above.

But if you want to use Amex points, you might want look into Delta, with is a partner of Amex. According to a web search I just did, Delta also flies from Kansas City to Bonaire, with the plane change in Atlanta rather than Miami. Btw, in hurricane season, ATL is slightly lower risk of connection problems than MIA. You don't want to have your flight cancelled if a hurricane is affecting south Florida but not affecting your final destination of Bonaire.

Just keep in mind that what you think of as "first class" on these shorter North American flights is actually called business class from a booking perspective. It's the new same for the bigger seats up front. Delta's up-front seats are not that much different from American's up-front seats on these flights.