Do it asap!! It gets sooooo much a harder once they’re mobile - then they don’t want to sit that long!
You’ll have to play with it a bit to see what works for your kid. My kids napped well in the car seat at that age, so I would probably have left in the morning, about 20 mins before 1st nap. Have one parent sit in the back and keep baby awake a few minutes, then at nap time, lower the shade on the seat and hope she naps for two hours. Then, see if you can vamp a little more. If she’s bottle fed, you can feed her in the car seat. See if you can get 3+ hours in the car. Then stop for a long lunch. Find a place you can lay out a blanket do she can wiggle, feed her, hold her. You guys eat. Back in the car, again 15 mins before nap time, repeat. If you get 3 hrs under your belt for each nap, you can do a shorter pit stop and then power through the last hour. That’d be my approach for the first one, anyway.
You’ll figure it out, but the only way it’ll get less intimidating is to try it!
The hardest time for car travel, IMHO, is 10-18 months. Get a visit under your belts now!