> Most senior folks in Spain don't work for Spanish companies in general; they work for EU companies that pay more.
I doubt that statement. As a German living in Spain since the pandemic, I dealt with this and my social bubble is full of tech expats dealing with this, too. Living in one EU country and working remotely as an employee for another one, is almost legally impossible, and full of unnecessary hurdles for both employer and employee. Everyone of top-talent grade I know who does this, is basically either self-employed (as I am) or operates a "legal construct" such as having empty "mailbox flats" in the country they work for (breaking all sorts of laws by doing so).
The EU itself has never harmonized income tax laws; actually all of the EUs political system always tries not to touch the tax subject at all. As a result, each EU state has an individual double-tax treaty with all other members states. Yes, do the math - there are hundreds tax treaties between EU member states. Non of them are based on some EU guidance or blueprint, and oftentimes older than the EU itself (german-spain treaty dates back mostly to the 1960s with some minor additions in the early 2000s). Finding legal advice alone is almost impossible (i.e. a lawyer that speaks either of your languages and both legal systems recently well). And if you do, good luck, your fellow civil cervant at your tax offices will screw up your fringe case anyways.
Worker-protection laws apply by country of residence, but the employer is bound by their national ones too. If you live in Spain and work remotely for a german company, legally you are bound by spanish worker laws. That is, you get spanish bank holidays off, minimum wage laws of Spain (and Germany!) and so forth. Even when figuring out all legal subtleties, it is simply not manageable for any companies HR department to deal with all country specific regulations and changes, let alone in different languages. I run a company myself and could not employ a person from another EU country within reasonable effort; the only way to go is hire them as contractors or through payrolling agencies. Both will not make them your employees, which has a lot of other legal consequences (holidays, employee patent/inventions laws, but also stuff like you can't really enforce any policy on them without going through the intermediaries).