What preparations? Only thing you have to do is ally the electors not allied with Austria, you can even declare on one of them to lessen the numbers of alliances. It is only if you for some reason care about the dip loss that it gets annoying to do.1) I also lack the necessary kind of patience to make the required arrangements to dismantle the HRE as an outsider.
2) Vienna's a coalition-bomb even without the HRE AE multiplier, because it is a very rich German Catholic province within the first AE range band of lots of German Catholics. (OK, it gets a bit milder after the reformation.)
And that is why I said to attack as they join? Only reasons to allow a coalition to build is if the first to join is someone you can't take on or if you aren't going to expand that way ever.