I have played many games as the HRE the way to act greatly depends on what the other factions will do.

The best thing that can happen is to manage to conclude alliance with nearly everybody so that you can focus on building your empire. Taking Pomerania in the first turn is ok with the Vikings based in Saxony but there a chance it will rebel the following year. The risk gets down to about 6% if taxes are very low, you have a governor with reasonable dread and bring troops from another province (build a spearmen unit in Burgundy, move it to Lorraine, move one unit from Lorraine to Friesland, Saxony etc so that you get one hundred additional men in Pomerania).

War with the French is not too bad since they are usually not allied with anybody. Since your alliance remain, you can concentrate your troops in the west.

War with Poland, Hungary, Italy or Denmark is more tricky since it triggers a cancellation in chain of alliances and, thus, the risk of a war on all fronts. In that case, my tactic is to stop any building construction and raise armies of spearmen. Even though they are not very good units they, at least, hold their ground against peasants, archer and light cavalry. Since you are unlikely to face anything else you should do ok unless a loyalist revolt takes place.

Hope it helps