I just thought about a few things reading through what's here.
1) I really think maybe you should give a look through this thread a little Steve. These 'what do I do now' feelings have basically been going on since D was gearing up to head off to school so I really think you need to sit with that.
2) You said the relationship you guys had when it was just the two of you was pretty rocky. Was it rocky because of her, because of you or because of you guys feeding off each other? Because of that past do you even know how to be a couple without being parents first? I'm not talking planned date nights and differentiation. I'm talking do you guys know how to be in a mutually beneficial relationship with each other without your D there as a touch point and a buffer?
3) As your daughter doesn't need you "any more" (that's in quotes because let's be real they'll always need us) are you feeling a little useless or aimless without her as the target? Are you feeling a little less grounded without anyone depending on you? Is there a chance there's a co-dependency thing happening here? Creating drama where there is none so you have something to fix? Please don't take that as accusatory. I have no idea if that's what's happening here. I'm just spit balling with what I know. And if you can cross that off the list of possibilities immediately then that's all I was looking for with that.
4) Given the above you may want to consider not only IC but a serious R talk. Maybe not force this as a crisis issue. Because it's not. You guys aren't in crisis. You as an individual are kind of teetering on the edge of one right now though. You might need to consider MC? A couple retreat? Some kind of reconnection reading or bootcamp? I'm not sure. But I'm wondering if it isn't so much malaise as it is you're floundering navigating a moment in your life that you've never had to do before and it seems easier and more efficient if you just called it quits and give yourself the opportunity to start over.
I'm not sure, Steve. I know you're not going to rush to act. That's not your style. I just think there's a lot to unpack here and the surface feeling of it all that you're having a hard time pinpointing feels very much like it's literally just the surface of something here.