Consumer Cellular uses either the AT&T network or T-Mobile network. Your SIM card determines which network your phone will use as a CC customer. Consumer Cellular phones will only work outside of the US if they're using the T-Mobile network, and as you noted, it's expensive. CC phones with an AT&T SIM won't work at all outside of the US.

You've got a lot of choices. One easy way is to switch from Consumer Cellular to T-Mobile (not just the SIM, but you must become a T-Mobile customer). T-Mobile gives you free data outside the US, and calls would be a lot cheaper than if you're a CC customer with a T-Mobile SIM. Or, you can get a German SIM & plan upon arrival. Google Fi may be another solution. But if you're leaving in just a few days, you may have to get something upon arrival in Germany.