I recently became emperor as Spain (though I achieved it at 1760). I wouldn't recommend it for you to try to get colonialism and emperorship. First of all, HRE politics are a huge pain in the ass. The current emperor has several modifiers to keep the emperorship; for example, big nation of the empire (+25 reasons), imperial authority (+1 reason per point), same culture (+10 reasons, I believe), diplomatic reputation (being emperor grants +1, which becomes +10 reasons for the electors). Besides that, in case of a tie, the emperor wins the election. Therefore, besides the alliances, those alliances must be partnered with +200 relations, huge diplomatic reputation, and even then it might not be enough. In my run, Provence was an elector with whom I was allied at +200 relations, but even then, he would not vote for me, as he was allied with the current emperor Bohemia. Thus, I had to use great power interactions to break their alliance, and I barely could do it, even if I was more powerful than the other great powers together. If Austria is still the emperor in your run, it will be still more difficult, as Austria's mission tree gives them bonus that help them keep the emperorship.
Also, the HRE after some time becomes a huge mess. After you become emperor as Castile/Spain you are likely a big nation, so most external nations won't attack the HRE, as you will defend them. This might seem good, but every answered called to arms earns you +10 IA, which are a big boost. Heretical princes after the age of reformation are a drain of IA. They require a lot of investment in relations, and even then, the princes might be reluctant to convert to your religion. It is even worse if the heretics are free cities, as you cannot even force convert them without losing 10 IA or taking a stability hit.
If you have not taken diplo/influence ideas, you will find almost impossible to become emperor. These groups provide you with good bonuses (diplo rep, relations slots, extra diplomats, etc.) If you have followed Castile's mission tree, you should have a PU over Portugal and one over Austria (or alliance+RM, for the chance of getting the Netherlands). That means 2 of 4 slots used. If you ally the 4 electors, you will run with 2 dip rep points per turn, probably for the rest of the game.
Now, consider what happens if you lose the emperorship. If you join the empire when you become emperor, and then you lose the emperorship, you will become a duchy, and suddenly, you will be way over your governing capacity. Then, you cannot afford it, so you'll need to keep the electors (preferably the ones that elected you) happy, so you cannot unocuppy their slots. Their alliances are fragile and can be broken by a war or otherwise. If you die without an adult succesor, you lose the emperorship. You'll need to fight France/Poland/Ottomans/Sweden frequently to protect the empire or risk losing 10 IA.
While revoking the privilegia might look attractive, you might have a hard time reaching it, especially if not a lot of reforms have passed. I think it is not worth the hassle.
