We use Kafka streams as event sourcing for the last 5 years on a production fin tech product. We use it in a similar way that you did and it works great for us. It does have it's problems, which you touch upon, but we've solved all of them except rolling upgrades, which seems to be impossible with Kafka streams. The weakest part of Kafka is Confluent. Instead of focusing on new features they've become a marketing company.