So yeah I guess other moves need to be done by respective GMs to figure out the cap but in general I think the trade makes sense.

Columbus:
Upgrade at the center position from Roslovic to Miller. A line of Gaudreau-Miller-Laine would be elite
Dermott could fill in on the right side with the departure of Peeke (I know Peeke is much better). They also drafted Jiricek who plays the right side and will likely be in the NHL by 2024 (maybe even this year).
Also have other young RHD like Ceulemans, Blankenburg, and others in the system. I think it makes sense as they just gave Gudbranson 4M/yr and I don't think it's to play behind Peeke and Boqvist.

Vancouver:
Gets a young defensively responsible RHD in Peeke to play with Hughes
Roslovic maintains their center depth and fits their age group. Three centers deep of Petersson, Horvat, and Roslovic is solid and will likely get the most out of their many wingers

I added a pick coming back to Vancouver but I wasn't really sure what it should be (i.e., what round and what year). I think some sort of asset coming back to vancouver on top of the players listed makes sense to even out the trade value but I think it can vary based on the perceived value of the listed players.

Also regarding a Miller extension, I think Columbus can make it work. Nyquist's 5.5M comes off the books after next season and could be used for the extension. I feel like with the signing big guns like Gaudreau, Miller, Werenski, Laine, etc. and the emergence of young guys like Sillinger, Johnson, Jiricek, they'll be a competitive team in the Metro in a couple years.

I would only accept this trade if it becomes clear that Miller isn't re-signing with Vancouver and nobody is offering anything better. Otherwise, it's hard to feel any incentive to accept without the 2nd being a 1st instead