It is NOT your responsibility to maintain the relationship between your exh and his children . Your only responsibility is to support your DC and to ensure that they can have a relationship with their dad if he can be bothered
I completely agree with this 100%. You are doing really well OP and I think your approach of saying, I’m always here for you and you can talk to me about anything, is exactly right.
I had one friend in particular who fell into the trap of making excuses for her ex. Please try not to fall into the habit of creating a fictional, engaged idealised Dad for the DC. It's incredibly damaging. It is tempting to say, "He's probably busy," or "He can't always answer his phone," or "I'm sure he didn't mean it." My friend did that to protect her children from being hurt, but making excuses just makes things more confusing. It damages the children and their sense of self worth later on. It can set children up to accept excuses from friends and partners who treat them badly and lull them into making excuses for unsatisfactory relationship partners in the future.
Children are better served by an honest reality. So it's fine to say, "Your Dad loves you in the way he's capable of." Make it clear that if he doesn't respond or doesn't show up, that's all about him and his issues, and that you will show up and that it is about him not about the DC.
Obviously don't demonise Twatoo, but the DC don't need to excuse every disappointment, making up an innocent explanations. If someone repeatedly lets them down, whether it's a parent or even a school friend, it's healthier for them to understand that this is a limitation in that person, not something they caused and that they deserve better.
Also you are responsible for your relationship with your DC. He is responsible for his relationship with them. Please don't try to tell him off or nag him into the communicative separated Dad you want him to be. You can't do that. Its not your job to manage his relationships for him.
It's much better to put your energy where it can make a difference. Gradually surround your children with good people who will show interest in them, grandparents, uncles, teachers, coaches, family friends and neighbours. I expressly told my DC that they had lots of role models; they benefit from having a network of adults who are kind, dependable and interested in them and that they don't need one perfect role model.
I realised that I didn't want my children's narrative to be, "My Dad kept on letting us down and he ruined our lives." Instead, I wanted the narrative to become, "Yes, my Dad had his issues and wasn't very engaged, but we had lots of other adults who loved us, supported us and were there for us so it didn’t make much difference.”