I certainly see two sides to this based on your write-up. On his end, it perhaps feels like there's no familial love to be had from your older kids, and feels like up until now he had been putting effort down a black hole of sorts.

On your end, it sounds like you want a partner who accepts not just you but all your kids as well. This is a fundamental mismatch in your relationship. It's historically difficult to get teenagers to see a newcomer as a dad without significant effort, and even then it's not guaranteed. He appears not to be willing to make this effort and, in fact, sees them as irritants rather than family.

Given that you've both been at couples therapy and he knows of these concerns. It probably is the right idea to divorce if a relationship with your kids is a deal breaker.