My dh’s family were and are, very sadly, pathologically monstrous. His mother was a hideous bully. But we were able to survive that by judicious distancing, putting clear boundaries in place, especially with regard to our dc.
The part I don’t think is as easy to negotiate is a partner’s relationship with their children. I don’t have personal experience of this, but I just wouldn’t get involved. There’s no point. Of course your advice on this issue is not welcomed and sorry but it’s right your dp is putting his dc first, at least until the age of 25 or so!
Of course 19 and 15 year olds are going to be mean to their father! Even more so having been through a parental divorce. And of course they are going to be emotionally draining. It’s naive to think they wouldn’t be! They are adolescents! It’s normal and healthy that they are being oppositional! They have to withdraw, question, and challenge their parents and their values etc in order to become separate individuals.
Honestly, parenting teens can be extremely difficult and you just don’t know how hard it is until you have been through it. It’s very, very good he is such an involved father as they really need him at this age.
The adolescent difficulties will resolve themselves but I’d say you have a good ten more years to go before they do.
I think you are very wise to maintain a separate home. Your dp sounds like a good bloke but often love isn’t enough on its own to sustain a relationship. He sounds as though he is being pulled in all directions atm.
In a nutshell, most relationships have to go somewhere and grow. I think the future of this relationship depends on what you are happy to accept in the future.
Good luck. It sounds as if you are weighing up whether his obvious good points are enough to outweigh the crappy elements of the relationship. Personally I think it would be worse to be with a man who didn’t fight for his dc but with the fighting comes a lot of attendant hassle and financial constraints and frankly it’s not your battle.