1. Most probably one will have to give up the old citizenship in order to benefit of these countries.
Yes, the process is a little more complicated than just finding a rent over there and moving out. Some of the countries also have their citizenship easier to obtain than others. To me it seems that putting in the efforts to revoke your citizenship and obtain another one just for the taxes only makes sense when you have a big bag of money that is worth the time and effort.
Can confirm Portugal also seems to be a good choice tax-wise.
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Regards,
PrivacyG