Speaking of water problems and Uraguay…
This indicates that about 56% of people in Luxembourg speak English as a second language:
You’re in the middle of Europe, the temperature is reasonable, it’s not as expensive as Monaco, etc. Seems like a stand-out option.
Those of you saying Malta are thinking too big. You need to go with the Sovereign Military Order of Malta.
Seriously, somewhere in the European Union is probably a good choice, because it gives you easy access to a lot of big-country amenities, if you so choose.
It’s the only country in Europe to legalize marijuana, that’s an interesting tidbit. Otherwise it’s quite religious, but Catholic not Protestant or Islamic, which is a different and more relaxed environment, YMMV.
Malta is the fifth densest country in the world, as are many other microstates up there. Not for me, personally.
Slovenia or Croatia? (I think Croatia is above OP’s 3 million limit, although not by an order of magnitude or anything, and Slovenia certainly isn’t).
Luxembourg is currently working on this. There was momentum in 2018 and 2019, with several intent-to-move-forward type laws passed, but then neighboring Germany freaked out and immediately thereafter the pandemic hit so the effort stalled.
Now, though, things are getting back on track.
(Germany has made no secret of its intent to freak out again, however.)
Nah, it’s got the highest concentration of microstates in the world: Andorra (OK, a bit of a stretch), Malta, Monaco, Vatican City (fancy becoming a cardinal? It might be fun to try 
), San Marino, Cyprus, as well as the aforementioned Malta. I expect Montenegro, Kosovo, and North Macedonia might be OK on the population criterion too.
Why stop at cardinal? I’d live there if I could be the guy who wears the red shoes and has a fancy ring and gets to set the rules.
I take it cost of living isn’t really a factor here? Some of the places mentioned are hellishly expensive.
I’ve considered moving to Costa Rica, Guatemala, and Belize.
Costa Rica, Jamaica, Bahamas or Netherlands for me.
Some people said the Netherlands, and that was my first choice too when I only read the thread title, but although it’s a small country, it’s very densely populated with a population of 17 million, and carries similar colonial baggage that the OP wanted to avoid. Depending on how long you will live, you’ll also run into massive troubles with rising ocean levels.
Re Liechtenstein: nice country and landscape for sure, but one of the most expensive places in the world. Anecdotal evidence: about ten years ago, a friend of mine was on a business trip in Liechtenstein and had forgotten to pack a toothbrush and paste. He went to the nest drugstore and paid freaking €10 for both. Other than the high costs, for me the killer argument against living there is the fact that it’s a full-blown monarchy, and I couldn’t live in one at all.
Monaco popped into my mind at first, but having grown up in southern California, I’m kinda burned out on the Mediterranean climate.
Considering that I love winter and cold weather in general, Iceland would probably suit me just fine - long long days in summer, long long nights in winter, and chilly without being apocalyptically cold.
Is anyone considering wether the countries they’re choosing want immigrants?
Costa Rica.
Or perhaps Ambergris Caye, a small island off of Belize. Almost bought property there once. A bit of a problem is Belize City on the mainland is a bit of a slum. At least the parts I saw.
If we want to follow the OP’s guideline strictly, Costa Rica is not an option with a population over 5 million. Guatamala is 17 million.
I was going to say Wales but then I saw the population cutoff for the thread. I know it’s more a nation-state than an independent country, but hopefully in my lifetime (Cofiwch Dryweryn!). I fell in love with Cardiff the first time I visited, and could easily enjoy retiring there. If I wanted a smaller town with great scenery, there’s always Llandudno on the north coast.
Sticking within the parameters, I suppose the Isle of Man (with a population of 85,000 or so) could do, though I imagine I’d take the ferry to Liverpool quite often.
No, that wasn’t part of the OP’s setup so didn’t consider it.
OP wanted to avoid “a history of colonial crimes”. It’s not quite the same thing, but people moving from the US or Europe to South America or Africa can carry it’s own quasi-colonial baggage.
I think he was talking about England’s legacy though. Not quasi-colonial baggage.