MN is weirdly bah humbug about proposals and these kind of threads always bring out this type of poster.
Meanwhile, in the real normal world, it's perfectly possible within a couple to have a general discussion about whether marriage is something you want - along with would you want kids / how many / when; where do you see yourself living / working .... whatever. Just general conversations couples have to see if they're on the same page.
Similarly - any man in a relationship should instinctively know the type of proposal his partner would want - so he will just do that. If she's shy, he's hardly going to make a big hoopla. He'll just ask her privately.
I mean, how hard can this be?
On MN, it either gets presented as this dramatic divide between "princesses" waiting for years on end; or these very matter of fact, "I just sat him at the table" types. As if there's nothing in between!
Also, any type of proposal is assumed to be naff and cringe or "Disney" - with all sorts of doom and gloom forecasts about "the type of man who proposes."
The fact is, most men do propose in some way and it doesn't need to be a spectacle at all. It's personal!
When my husband proposed, he got down on one knee on some cliffs in Italy. It's wasn't cringe and it wasn't a scene for anyone else or Instagram or whatever. I was delighted, but it wasn't exactly total shock. Similarly, I don't think he would have asked if he thought I'd say no! Anyway, it's a lovely memory to have and we've since taken out kids to that place. What's wrong with that and why shouldn't a man make the effort?
OP, I think you're worrying about nothing. Your DD will probably just initiate a conversation about the future in general. He'll probably propose in the near future. No drama.