Hey Sluggo, good to see you again. It's good that you're thinking through options ahead of time.
I might make a distinction between getting the kids' "buy-in", vs getting "feedback" from them &/or "letting them know and being open to their perspective".
A concern that comes to my mind in these situations is that there can be tacit expectations on the kids about how they should respond. I'm definitely not saying this is going on with you guys, more sharing my observations on how I've seen it go. For example, a parent/stepparent, when wanting to marry a new partner, might say to the kids "Kids, I'm going to ask your mom to marry me, but I'm also going to respect you by asking you too -- will you take me as your new stepdad?"
Stuff like that can seem caring, respectful, treating the kids as "grown up", etc, but what it can really do is put the kids in a corner. They know Mom and Mom's Boyfriend want to get married, and so if the kids say anything honest in response to the question, like "Um, actually No", then Mom will be sad and it's the kid's fault. So those kinds of questions aren't really honest questions, because the adults are going to do what they want, and are asking that question to seem like "the good guy who really cared about what the kids thought".
This is basically how it went down with DH's kids' mom and stepdad -- from what I recall, he "asked the kids to marry their mom"... the kids were like 4 & 6. I mean, what can they say? No? There was a tacit expectation to "make mom and Stepdad happy" and to play along with a dishonest question (by dishonest question I mean, there was no chance that if the kids said "Actually I'm not ready for you to get married", that the adults would've slowed down).
So, if it were me, I'd steer away from any kind of question that frames the kids as having a say or a choice in it -- I'm assuming they don't. And, in fact, it isn't healthy developmentally for kids to "have a say" in adult romantic relationships.
But I also don't think you're doing that. I'm guessing that when you say "get their buy-in" it's more getting feedback from them? That can work healthily as long as you're really clear about what's going on -- that you're not asking the kids "if they are OK with it" as that opens a door for them to say "No, I'm not OK with it" and (a) they have power over adult relationships they shouldn't, and (b) if the adult relationship still proceeds, they may feel "tricked".
I'm guessing that what you would be talking about with the kids would be phrased more like:
"Kids, I want to let you know that Girlfriend and I are talking about getting married. This wouldn't happen right away, it would probably be in about one year -- so, around 2024. One change that would happen is that she would start to live in this house, instead of in her house. What wouldn't change is that all of us would still live here and keep the rooms we have, and go to the same schools, and do the same activities. I'm wondering how you're feeling as you hear about that change that's going to happen?"
And then follow up questions/comments, like "Even if you don't have any questions for me now, or things you want me to know about, you might have questions or feelings later that you want to tell me, and I would love to hear about what you think and feel." Of course, tweaking it to match your kids' ages -- your 16YO can probably handle a more grown-up conversation than the rest, where you talk about resources your 16YO can seek out for support, people to talk to, counseling alone or with you if desired, etc.
I guess overall, if it were me, I'd frame it as -- kids, here is a thing
that is going to happen in our lives, and here are some facts about it: when it'll happen, what change you'll notice, and what won't change. What feelings and thoughts about that change do you want me to know?
I wouldn't phrase anything anywhere close to them having a say in if it happens or not. Keep your hands on the steering wheel of this one, with a huge open door for them to "give feedback about their feelings" and to ask questions.
How do you think they'd do with that approach?
Are there other concerns with a more "announcement" type approach (like, is there still enough conflict with their mom that they might pull a "you never let me decide anything, I'm running away to Mom's" move)?
Are any/all of them in counseling?
Hope that's helpful food for thought. And, congratulations!

kells76