To the Americans who are daydreaming about this I think it is actually very doable. As British citizens we were able to move here under EU freedom of movement, but don’t know what terms we will be under post brexit. To get a feel for the worst case I researched what we would have to do to be able to live here as a non EU citizen such as an American, and there are few routes to look at. For example I’m currently resident as an autonomo, effectively self employed, and there are visas available to self employed as well as entrepreneurs, either of which could suit someone with a side hustle.
There is also a non lucrative visa which doesn’t allow you to do paid work, but for which you basically have to prove that you have the resources to support yourself, and that you have private health insurance - but as this is Europe think in the region of EUR100-200 a month for insurance for a family of 4. The level of support seems deliberately vague but shouldn’t be a stretch for most mustachians, and as far as I can tell they are usually happy to see enough cash in a bank account to cover 12 months of expected expenditure.
Taxes wise Spain is certainly not as low tax as the US, but also far from the highest in Europe. Generally speaking the income tax brackets are pretty similar to the UK, although the higher rates kick in at lower levels of income than the Uk and there are less generous exemptions.
Another factor that we thought would be a deal breaker for Spain is that there is a wealth tax on assets above EUR 700k per adult, with a separate exemption for a family home. However there are a lot of exemptions to this, for example pension accounts dont seem to be counted, and investment property is counted at the original purchase price rather than current valuation. Overall this means that the wealth tax seems to be less of a concern for even a moderately mustachian stache.
All of that said if you are looking for somewhere nice in Europe to set up home and are otherwise undecided between Spain & Portugal, it might be well worth taking a look at Portugal. As I understand it they have a pretty advantageous tax treatment for newcomers whose assets are mostly outside of Portugal. But for us there were a number of specific factors such as location, and for kids to learn a more internationally useful language that made the trade off for Spain worthwhile for now.