I own two Amazon KDP accounts as well; first of all, I would say a few things:
1) Don't believe that since the "work is already done," you won't do any other job in the future, especially if you want to live off KDP, the market is becoming saturated, and the AI tools are not helping at all, also, all the new platforms and
marketplace boost new listing and content, so, if you don't create new books, your income will sharply decline after some time, so, don't believe that you are set and done, you will need to work if you want to maintain your income
2) When you talk about Royalties, people believe you are treated the same as an artist, this is not the case with amazon KDP in most countries; in Italy, for example, it is treated and taxed as an e-commerce rather than Royalties for IP, same in France and
Spain, so, yes theoretically you receive royalties but then the interpretation change from country to country
3) Since you need to run ads and since you are effectively working on publishing and creating the books, the place of effective management counts, so if you choose Portugal, you will need a nominee director to be compliant with the law. otherwise, you will need to pay taxes on a Portugal level
I consulted with quite a few people and one lawyer told me (freshportugal) that I can deduct expenses from a US LLC, so the total tax rate would be lower than the 10% since you'll get a credit from the US.... I am not sure that this is true since the withholding at the US Level, so I call it BS, so far I didn't find a better option that Portugal, even because I don't want to live in Georgia, another option would be cyprus with their 12.5% total tax rate... and I am now looking at Thailand, Philippines,
Malaysia or
Mexico, Domincan Republic or Honduras /caraiibbean to see what can I do to lower my taxes since I visited Portugal and It doesn't make me crazy in love