First, you might add Gerona north of Barcelona to your list.
You've gotten some tips from several people who have moved to Europe. I agree thoroughly with Nelly and others who have suggested you look on websites, but I suggest private Facebook groups of people already living in the areas you are considering. You'll find the most general such as Retiring in Such and Such with 200,000 members to Americans Living in Malaga and Costa del Sol with 552 members. There are even FB groups for obtaining visas, getting or exchanging a driver's license. You want to be near water, so it's already narrowed down to the Mediterranean or the Atlantic.
We moved to France because we are French. But to decide where, we explored different cities several summers with a list of criteria.Next came the criteria within the city. Information posted by one member on one of the Facebook groups led us to our area. We did see apartments before we were ready to move, and it was a waste of our time because we were exploring, not moving. So concentrate on choosing the city first. Once there in a short term rental, you can get yourself situated.
Right now we have a huge influx of Americans where we live. Their visas were not difficult to obtain, unlike Jo's visa. Different country, different type of visa, retiree vs self-employed. I'm not sure why some people see a visa as a big obstacle, when it’s not for a retiree. In fact, several hurdles have been mentioned, which don't exist for most people.
These are some of the things you'll learn using the Facebook groups.
You'll need a US address for a couple of things, one US phone number for two factor identification, most investment firms won't want to keep you unless you are in the 10million group so decide how to handle that while gone for a couple of years, read the tax treaties and the inheritance treaties just in case, you can have your Social Security directly deposited, cheaper than using Wise. Etc, etc, etc.