Retirement and residency - confused ?!?!

Am I being dumb ?
I'm at the early stages of researching Antigua as a potential retirement home for my wife and I. We are British and have enough assets/funds to buy a good sized house and retain some UK property for residual income.
I am 54 now and will also have a fairly good pension avaiable after 55. We have no intention or need to work if we did retire to Antigua.
So we're in a reasonable position financially, but I can't easliy work out whether we'd be allowed to live there.
The residency permit seems to need 5 years legal residency, which I can't see how you can get without the residency in the first place.
I have read about the citizenship by investment, which I think would require us to invest in a (rental) property on the island at least to $400,000. Whilst we might be able to do this (by selling up completely in the UK), it would probably reduce our rental income, so not attractive.
Can anyone provide some advice on what's possible ??
Thank you

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