Bonding buns never seems to go quickly. I read once that you come to a point where you think “Oy – This has been going on FOREVER”, then suddenly – they’re bonded.
They may not need the stress bond car ride. I am unsure from your write up if the lunges or “head bounces” are aggressive or normal dominance behaviors. While I was super nervous about Jackson and Coco Doing their dominance moves on one another for fear it lead to aggression, I also know it is something that HAS to occur between the two. Perhaps your nervousness is causing them to both be nervous? Are you mis-interpreting the need for the two of them to sort out dominance as aggression?
Honestly, Coco ( my female) was so much savvier at what she would tolerate from Jackson ( my male) once I reflect back on it. She really wanted to love him, but she never allowed him to mount her willingly. I have seen her mount him recently. I worried that if he mounted her, she would freak out. She simply never let it happen. She would move away from him if he made an attempt.
They spent much of their bonding times having him chase her. Not aggressively, but constantly. As long as they didn’t become truly Aggressive, I would let them spend their full allotment of time together.
You are doing all the right things – switching pens, cages near each other. You are just going to have to be a super keen observer of their individual body language styles to sense if they are about to throw down in an unholy row – OR figuring out who is going to be the boss.