First of all, I am not planning to adopt archduchy as government
That's a mistake in my opinion. Austria's strength is its diplomatic relations. An extra +1 relation might not seem like much, but early on it's extremely useful. As is the -0.1 monthly autonomy change.
First of all,just how aggressive should I be in this campaign?Am I only supposed to expand via establish personal union cbs that come in the 1500s?
One of the first things you should do is royal marriage Bohemia (Don't ally though). Because they start in landfriede, it's extremely easy to get a Habsburg on their throne. I'd say 9/10 times that works.
Once you get your dynasty on their throne, immediately claim it, then declare war once you get the CB. It will probably be a tough war, but if you play it right, you shouldn't lose too much manpower (use mercs to bolster your infantry and let them attack you). Once you have enough warscore (usually 60% or so), force the union.
It's a good idea to raise relations with electors and surrounding powers beforehand in order to minimize coalitions and retaliations. Once you have Bohemia PU'd, you now have one guaranteed vote for emperorship. Maintaining control of the HRE is far easier now. You'll also inherit the electorship if you wait to inherit without explicitly integrating.
Bohemia makes for a strong subject and will help conserve your manpower. Just remember to bump relations above 0 after the war to maintain the union.
Another thing is am I supposed to intervene actively and try and release annexed HRE states as much as possible?I tried fighting these wars but ended up getting my manpower really drained.
I wouldn't worry too much about that. Sometimes it can be useful to let a few smaller princes get annexed. Then you can free lots in one go. Just be sure to choose your intervention wars carefully so as not to engage strong alliances right at the beginning. It's not imperative to release states straight away, even if the CB timer is about to elapse. It's easy enough to find a pretext later on. What you need to keep an eye on is the core timers. Once they disappear, you can no longer free them.
Enforce peace and guarantees can be useful if you want to help protect certain smaller states from annexation. On that note, it's worth mentioning that intervening in foreign wars can provide significant IA.
Secondly,I am supposed to post diplomats to try and establish good relationships with most prince electors?
You only need to match the most votes to maintain emperorship. Two or three is usually enough. The more IA you have, the easier it is held. If you PU Bohemia early, you'll probably only need one other vote to keep it. Check the HRE screen to find out how many votes you need. Theocracies are best ones to go for usually. Raise relations, send gifts, bestow imperial grace, etc. Giving them back their territory, releasing them, and enforcing peace against counties who attack them will help immensely.
Another thing is who should I enter alliances and royal marriages with?Just the electors or should I ally with a foreign maritime power like Castile?I intend to obliterate Venice in the near future but I need a fleet to do so.
Castile can make a good ally, but it's not necessary if you PU Bohemia early. Personally, I prefer to ally potential vassals like Baden, Urbino, and East Frisia at the start.
Serbia makes a good march if you intervene and force vassalise them when they attack Bosnia. You can also get Byzantium as a vassal from that war in most cases. Prepare by raising relations with Bosnia near the start of the game.
Another strong potential vassal is the Teutonic Order. If you get the opportunity (while they're engaged other wars/busy), take Neumark from them, core it (or leave if the OE doesn't bother you), then wait for Poland et al to fully annex them. Release as vassal and proceed to return cores at a time of your choosing.
Another thing is should I move my capital to the lower countries if the Burgundian Succession event fires?
I usually just move my main trading port there, but you can if you wish.
Final thing is if I decide that it's better off to release Burgundy as a vassal,would the Netherlands independence event still fire?
I'd caution against releasing as a vassal if you're still small because you might end up with a strong rebellious subject. Unless you're only talking about a few provinces that is. I'm not sure if the events will still fire. You'd have to check the wiki or something.
