What are some ways you navigate the pivot from engaging with colleagues from a place of collaboration and ideation towards a stance of execution and accountability as deadlines approach?

We're actually in the middle of a project like that. For major launches with large cross-functional teams, there is a point of transition when you, as you say "pivot towards a stance of execution". Some thoughts on how to prepare for that point and then make the transition. I'm putting this is sequence mentally on the fly, you be the judge of how robust or not that is ;-)

1. Get clear on the objective of the project and how it ties into the overall strategy. The why and the what. I like "Good strategy, bad strategy" as a framework
2. Get broad input from all relevant stakeholders. Refine the objective, play it back to people to see if you really are on the same page.
3. Pull together a small working team to define the overarching go-to-market plan and a high-level project plan with swim lanes for execution plus presumptive owners of each lane
4. Determine hard dates and dependencies and ensure the high-level project plan takes them into account
5. Get buy-in on go-to-market plan

At this point you should have a deep understanding of the goal, a solid plan for how to achieve it, and a small team that can help you ampllify your message internally. Now's the time to get $#*+ done.

6. Pull the entire go-to-market group together for a kick-off. Everyone who needs to do or guide work as part of the executuion. Share the what and the why combined with the go-to-market plan and the high-level project plan. Call out the owners of each swim lane and leave ample times for questions for them. Reinforce that the goal of the meeting is to inform and align but especially to assign clear ownership of each swim lane. Give the owners of each swim lane ample time to ask questions and articulate where they are missing information. Make it clar that your expectation is that each owner now takes on the accountability for their swim lane, details the project plan, proactively seeks clarification, and will report on progress

7. Set up a recurring schedule between now and the project end date to bring this group together to report on prgress, elevate blockers, make decisions, and keep things moving

8. Break into smaller groups and workshops for anything that gets stuck or isn't moving quickly enough. Work synchronously on those items with everyone in a virtual/hybrid/whatever room. The goal is to accelerate the process by cutting out the wait times that sequential work causes. Do this only for what needs attention so you don't drive everyone nuts.

9. Deliver on time.

10. Celebrate

I'm sure there are some giant holes in this answer. You could write a book on this and I'm sure there are many out there. Oh well, it was the best off the cuff summary I could think of ;-)