Best tips for an emperor gameplay?

Hey, I'm around year 1400 playing as Bohemia and I grew to be a local hegemon with no real threat around me. I enjoy the game politically so far.

However, I'm puzzled with the development of HRE and I'm worried to perform certain actions as I can't undo them on Ironman.

So far I've noticed:
- top1 thing to do is to pass Cities Contribution - Imperial Register under Military Laws. Essentially, around year 1370 it gives 700-800 manpower a month and a capacity of 50-60k which is HUGE compared to a baseline - almost none and saving for years for one professional regiment (those are times before armories). not sure if changing that to Electors Contribution would kill this huge benefit I get know
- it's very easy to lose a vote for a next emperor if you're in an union. I think it's a bug, but e.g. my current situation where electors are inside the union with me: Luxembourg, Upper Bavaria and Brandenbourg get their votes from other electors because... I am the dynasty there now. seems like electors are choosing my junior partners because they're basically me but without negative diplomatic relationship due to my annexing atm.
- imperial tax law is not worth it in an early game
- spying on Austria early on and killing Habsburg so now the throne in Austria is not within their reach was **I think** beneficial as I potentially avoided a lot of Habsburg related random events?
- I allied Hungary early game so they never attacked into HRE

but I'm puzzled about:
- is it beneficial for me to elevate most of the counties to duchies?
- what's the long term play around free cities?
- does it make sense to keep 2 electors under an union with me?
- is letting Brandenburg have their own dynasty and being independent more beneficial to the empire?
- is Austria a threat?
- how to increase military power and taxation even more?