Gospel. Final pot size and the veg routine leading up to it (because they're both linked!!!) is where you make your money and will be one of the main if not the main variable you'll need to dial in over time. Your flowering room is like a hospice. . . it's just where your plants go to die.
If you think less plants in larger pots is a change that's worth making, in order to do it right, you'll need to plan on it from the first day of veg.
Gotcha. Lesson learned. Nothing goes into that tent without a 3 gallon bag in the future (unless it's a long-flowering sativa, obviously).