"What makes me uneasy is that they had all kinds of plans, marriage and children etc and my daughter is very keen to have children and this, to me, leaves it all up in the air. He says he won't transition medically until after they have children"
"They are both nearly 20."
Very different situation, but I'd just like to bring in the experience of a relative of mine. Similar ages, the rest of the family didn't view it as a particularly serious relationship for her, I genuinely don't think it really was. But boyfriend got sick, hospitalised for months, and you can't really chuck someone in those circumstances, can you? Kick a man when he's down etc. So, the relationship limped on, and he never fully recovered, and she continued to not chuck him, and got into the habit of being his support. I think circumstances bounced her into marrying him, and nobody was surprised when the marriage lasted less than a year.
This sprang to mind because I could see a young woman being bounced into being her wannabe-trans boyfriend's support system and all-round skivvy. Because she wants to be a good person, and just like my relative, a factor has been introduced that will make it difficult for her to chuck him and not feel like a bad person. Because at 19, relationships do fizzle out naturally. If he hadn't brought this into the mix, who's to say they would still be together by the time they graduated?
I'd be concerned that this could form a glue that keeps her with him beyond the natural limit of the relationship.