If Brightline can get people to wait on the right part of the platform, as for example the Japanese do succesfully, and all doors are used, station dwell times can be kept quite short. Add to this the good acceleration of trains, and maybe it will be possible to insert additional stops without there being serious repercussions on end to end journey times.
Brightline has Car, Door Position with list of seats closest to that door, marked on the platform.
The problem is training people on the train that are detraining to get ready by the door to disembark before the train gets to the station, rather than lethargically getting out of their seat and start looking for their children and baggage after the train has come to a stop at the station.
A station stop like in Fort Lauderdale which in any rail literate country/area would have a dwell time of one to two minutes at most, has a dwell time of 5 mins that is often not sufficient in the rail illiterate country/region like ours. Of course being in Florida does not help either. On the NEC people understand the importance of getting off as soon as the train stops, but not in South Florida yet.
To be fair, I think they're also accounting for a possibly higher share of ADA pax in the longer term (it is Florida, after all).