Other FLs and Members are more of the bonding experts here, but my understanding is that the process is more like 2 steps forward, 1 back, and your two sound like they are behaving that way. In other words, you bonding work IS progressing as it should, especially with grooming and snuggling (cute pix), but they are not really “finished” bonding. It takes time and patience, especially toward the end when you can start to be sure the bond is made. Even then, a little change, like a vet visit, can set them back.
But they aren’t set back all the way. Just a bit. So, I would say keep up the car rides, try a laundry basket on a vibrating dryer if you have one and put them both in that. It simulates car ride motion and they snuggle in the tight basket together. (Supervised by you of course.) Generally, you sound like you’re doing the right things, just need to do it for a bit longer. Hang in there. Perhaps someone else has another helpful tip. Be sure to read through some of the posts in the forum for similar bonds and other tips.