Every type of thing you have, whether money, or manpower, or admin points, or prestige etc. is a "resource" that can be transferred into other resources, or 'invested' for some kind of long-term return. For instance by hiring an advisor, you transfer money into that type of monarch point (and get the advisor bonus etc.)

It's just a matter of looking at your resources, and examining what it's possible to do with them. Then you just choose, whats the most important resource to get more of right now, and how do i get that with the highest roi?

The best way to get more money in the early game is to conquer more land faster. Then you can constantly be selling crownland every 5 years, be draining enemy treasuries, and of course be coring and diploannexing more development. The roi on conquest is simply far higher than any other kind of investment opportunity in the early game.

So for 1., the best way to get to a stable economy is to spend as much as possible on sucessful conquests. Then youll have a huge amount of crownland to sell, and development to recieve income from. For the HRE specifically, because of the ae penalty, you can try stacking ae reduction modifiers, like prestige, or hiring a diplomat, or taking humanist/diplomatic ideas. You can get more prestige by removing the rivals of minor allies (in seperate peaces, its just a peace option that gives no ae and a lot of prestige), and you can make sure to spend your ae efficiently by only taking land you have a cb for, and only taking land from cobelligerants, and/or by managing the number of valid coalition partners with truces and improve relations (cant join a coalition if less than 4 valid members, cant join if positive relations, cant join if they have a truce, cant join if less than 50 ae.)

For 2. Youll want to always be up to date on millitary tech because that will help you win wars. The other techs dont matter as much, but its good to take especially admin tech for the idea slots and governing capacity. Make sure to give out the monarch power estate privledges and hire advisors to get more monarch points to spend on conquests/techs. Devving provinces is usually a bad idea unless you have stacked a lot of modifers, since its better pay 10/8 power for a dev (the cost of coring) rather than pay ~50 for a dev (the cost of making a new dev.) You might want to dev up a province if you have no choice or need to get an institution.

1. When playing in the hre you either want to become the emperor or get rid of the empire entirely. Becoming the emperor will let you take all the hre members as your vassals for free, while getting rid of the empire will let you expand faster since there will be less ae and no penalties like unlawful territory. Either way, keep on the lookout for opportunities to get elected emperor and build IA, or to get rid of the empire in a war (by allying or occupying the capital of all the electors/emperor. Speaking of which, unless you are getting rid of the empire soon you will want to be the emperor or on good terms with him, since otherwise he will demand unlawful territory which dramatically slows down your conquests. keep him at +100 improve relations at all times and try for an alliance.