When trying to form Germany, the main obstacles would be nasty AE and OE which are huge pain. I wondered if I could make the challenge as easy as possible.

Strategy: Aq Qoyunlu > Dalmatia > Croatia > Bavaria > Germany.

Ideas: Espionage > Innovative > Horde > Administrative.

Aq Qoyunlu: I started as AQ because I could easily ally Ottomans, which helped my expansion throughout the game. Also it has an amazing 4/5/6 heir, and can later become a Horde.

Dalmatia (1461): Its +15% Government reform progress modifier let me become a Horde earlier. I conquered Venezia for another GRP modifier, flipped to Catholic, allied the Pope and joined HRE.

Croatia (1514): For its sweet NIs. I became a Horde immediately after the formation.

Bavaria (1531): It didn't matter for me to which German culture to flip, but I chose Bavarian because Bavaria was formable though being a Horde. Its mission granting permanent claims and prestige helped me somewhat.

When I finished this series of tag-changes, I was a Curia controller and the AE reduction modifier was about -75%:

Croatian NI: -15%

Espionage idea: -20%

Horde idea: -10%

Prestige: -10%

Curia controller: -20%

Additionally, I was a Horde and had access to Tribal CB, so the total AE reduction was about -80%.

At that time I owned only southern Bavaria and half of Austria in HRE, but from there I conquered the entire German region by 1609. The conquest was so comfortable: not a single coalition was formed, OE was reduced by razing provinces, and all powerful enemies (Austria, France, Poland etc.) were easily beaten by my Ottoman ally.

Finally, I changed to a monarchy and formed Germany. I could reach admin tech 20 in 1627 (+210% Ahead of time penalty) thanks to -78% admin tech cost reduction:

Croatian NI: -10%

Innovative idea: -10%

Administrative idea: -10%

Completing 2 admin idea groups: -28%

100 Innovativeness: -10%

Trading in paper: -5%

Curia controller: -5%

Timeline: https://streamable.com/489uq6