Started my first Austria > HRE game after hearing being the emperor is like being a kindergarten teacher. Any tips about what I should look out for?
Make the Burgundians your waifu so you inherit their land instead of France or England, ensuring you reclaim the lands of the Empire. Then you only have to worry about an expansionist France (which if you are diplomatic, can usually avoid), allowing you to consolidate the Empire. You'll want to diplo idea up to spread your Hapsburg tentacles everywhere and ensure your best chances of inheriting princedoms, and it may be worth guaranteeing the independence of Byzantium (maybe not Venice, as they will have territories you will want in future) just to slow the Ottomans down very much. It is a good idea to support the Polish against the Teutonic Knights, in order to preemptively destroy a powerful protestant Prussia from interfering with the HRE.
Your main issues will be dealing with electors (easy enough with alliances and marriages, though they can occasionally during periods of reform, hand over control to one of your rivals) and dealing with rivals (best not to fight them if they're within the Empire - only fight to humiliate them in front of the electors). At all times try to maintain peace within the Empire. If you nommed Burgundy then prepare for the Dutch, if not, ignore them.
When the reformation happens prepare for all of your shit to be fucked up. You will be better off as Catholic, as to convert to Protestantism will momentarily disqualify you for Imperial inheritance - it's a lot easier to weather the storm as a Catholic maintaining the status quo.