> The 12-month engineering plan that I built 6 months ago - quarterly plan, hiring plan, integration roadmap - is now irrelevant

The 12-month plan was always a fantasy. Even planning 3 months out is too slow. Things change, new information comes in, plans get updated. You have to plan, of course, but never try to stick to the plan.

All hail the prioritized backlog. Toss your ideas on the board, prioritize regularly, work from top down at your preferred velocity.

>never try to stick to the plan.

Yes. Again and again in my 15 year career so far, the effective leaders were those who understood that "stick to the plan" does not apply to project timelines, and the ineffective leaders were those who thought their job was to ask developers for estimates on every task and scold them every time that estimate was wrong, and that was their whole game plan.