“Franco squashed the 20th century trend to separatist nationalism, because he was a Spanish Nationalist, and because the separatists made deals with the desperate Republican government”
Squashed, but not killed.
I’ve often thought, and IIRC, this is a bit of a conventional idea in Spain, that Spain stayed together as long as it had an empire. There was something in it for all parts of Spain. When Spain lost the richest remnant of its empire, Cuba, in 1898 there wasn’t much reason left to stay together other than tarriff protections, under the conventional wisdom of the day. The EU changed that, but the resulting prosperity kept everyone happy enough for a couple of decades. But the good times went bad and the fundamental benefits of unity aren’t there anymore.