I doubt they have standing. They neither made any speech or were retaliated against. They also have no connection to RCID.
If we ignore the speech and retaliation issues completely, there are still other problems that can be looked at.
The easy group with a clear standing are the constituents who previously voted to elect the board. Putting blinders on and ignoring everything else, that group has clearly been impacted. One could argue home rule and FL constitutional problems with the change.
In prior years, the tax rate paid to RCID must have changed as the district needed more or less funds. At least for amounts not financed through bonds. When they made those changes in the tax rate, would the board make that decision or did they put that specific change to a vote of all the constituents?
If the board can jus set the tax rate, and they're no longer voted for, that certainly sounds like a straight forward impact and issue open to litigation.
If the board just recommends a new number and then it is put to a general vote, then it is more nuanced. I can imagine arguments around exactly which things are subject to home rule and which are not. That someone would try and split them up using an argument that constituents still get to vote on all those things and the board is only directly impacting other things that do not disenfranchise constituents. I don't agree with this, but it is an argument I could imagine being presented.
As with all things, the nuance and the specific details all matter and are important.