You will only be able to soak up Germany's troops if they allow your crusade to pass through their territory (by the time you read this, you'll have found out about this). So it helps if they're allied or at least neutral to you.

If you're actually in the midst of a war with them, then they will fight off your crusade every step of the way. You might start off with 2-3000 men but you could be down to a few hundreds before you get out the other side of Germany, so it'll be a damp squib if it ever gets as far as the Holy Land.

In my HRE GA game, the English crusade to Tripoli ended up fighting against the Byzantines, who wouldn't let it through unhindered so it died a death, in Greece. Expect a similar reception for yours. Once into Orthodox territory, your desertion rate will increase, so it always helps to use ship lanes to get it to its target province as quickly as possible after its size has peaked. If you don't have enough ships yet, then scout the lands it will pass through BEFORE you launch it.

The best bet for your faction's first crusade is a target relatively nearby. If the German's get excommunicated, target a province of theirs. Your king's influence will get a boost if it succeeds and this will buffer you against loss of influence if a subsequent Crusade to the Holy Lands should fail, thanks to having to fight every step of the way, overland.

Note that you can't launch any further crusades until one has either succeeded in taking its target or has ended in failure. When they do fail, they take years to shed their last remaining troops, if left to fade naturally (and you lose the 'disband' button on units within a crusade). You may have to make 'suicide' attacks with the marker until the last of them has gone.