Holland is a better Capital than anything in Iberia, it's high dev and in argueably the best trade node. You shouldn't move your capital as Castile, that's true. You won't control enough of the British isles to make the English Channel really worthwhile early in the game and forming Spain moves the Capital back (thus wasting your adm). But you should move it as Spain. You can form Spain 20 years before the revolts start, so you don't need to move it while being Castile.
Delaying forming Spain is bad. Sure it costs you a bit of total force limit, but Aragon is always a paper tiger. They don't have good National ideas for combat, they rarely pick any idea groops (90% of the time they go Religious->Trade) and they seem to fall behind in mil tech sometimes. Forming Spain not only makes their troops as good as yours but it makes yours even better (with the splendor ability)
Annexing the Netherlands works, but it's quite a lot of AE in a place where you don't want it. First it's one route of expansion (France->England->Scandinavia->Russia) and second it annoys the HRE (especially the electors). So if you want to become emperor avoiding unnecessary wars there is helpful.
Delaying forming Spain is bad. Sure it costs you a bit of total force limit, but Aragon is always a paper tiger. They don't have good National ideas for combat, they rarely pick any idea groops (90% of the time they go Religious->Trade) and they seem to fall behind in mil tech sometimes. Forming Spain not only makes their troops as good as yours but it makes yours even better (with the splendor ability)
Annexing the Netherlands works, but it's quite a lot of AE in a place where you don't want it. First it's one route of expansion (France->England->Scandinavia->Russia) and second it annoys the HRE (especially the electors). So if you want to become emperor avoiding unnecessary wars there is helpful.