PREFACE: I am no bunny expert compared to others on here. I, myself, am going through my own bunny bonding issues right now.
How long has it been since they have been separated? I’m confident that you could probably rebond them. However, they are both definitely going to be neutered. From what I have heard, and from what I have experienced with my first male rabbit, neutering will definitely calm down Bombur. Hopefully, it will also take down his desire to be such a dominate “top bun.” In addition, I have also heard that neutering doesn’t have a huge effect on their personalities. Neutering will most definitely make them both more mild buns and without hormones in the way, I would guess that Asriel will continue to grow and be confident.
Regardless, neutering is good for the overall wellbeing of both buns no matter if they end up being bonded again or not. I would advise against getting a female though if you’re worrying about Bombur being over-dominating. I contemplated the same thing with my two boys. I believe that adding a honey-bunny would only increase the chances of them fighting if they are not bonded prior to the female bun’s introduction.
Again, I am no expert but I hope this helps.
Best of luck!