"Die Tiefe der Zeit" by Andreas Brandhorst.

Title translates to "the depths of time." I don't believe it's been translated to English.

I'm only about 10% in, but I absolutely love it. Futuristic setting about 10k years in the future. Humanity won a fight against an alien species only to later go into war with another, which has lasted thousands of years.

We've engineered ourselves to survive on various worlds, but something is going wrong. We seem to be getting less fertile.

"Great Mothers" lead space ships. They live for hundreds of years and have thousands of children (grown artificially).

But now the enemy, The Crul, seems to be coming out of wormholes (on journeys that took thousands of years) just in order to fight humanity.