Thanks for that. I have now talked to multiple people and it seems that reflagging is valuable when the new flag has less onerous obligations than the current
flag. Keeping the Spanish flag would cost me more money
than reflagging as USA (or Dutch for that matter). As a US citizen, I'm familiar with flagging as US Coast Guard documented boat, so I'm comfortable with that. I also have a person in Florida
who can handle the reflagging for me, so I have it all buttoned up.
I've learned a lot about VAT, flagging and Schengen in the last week. This forum has been great in getting this figured out, or making sure I'm asking the right questions.