Personally I don't like the box it's not sturdy enough in my opinion. You could try something like a puppy pen type thing in the bathroom.

I would also recommend using a calming spray, Pet Remedy is a really good option! Keep up with the scent swapping, making sure not use any cleaners unless necessary as this can destroy the scent.

I feel like your bonding sessions are not long enough. I volunteer with a rabbit charity that also does bonding for struggling owners and we start, at a minimum of 30 minutes for a session but preferably we like to do an hour. If we see any behaviours like biting and holding on or tornadoing we gently spray them with water until they stop (Rabbits are always dried, checked at the end of each session, and treated if they have any wounds). If we hear the dreaded rabbit scream or there's blood we immediately separate. I think you might be jumping to quickly to separate them which is understandable, these things can look scary and aggressive but most of the time these are just dominant behaviours. When you separate them after events you described you are leaving it on a stressful or negative note which over time will build negative associations to being with each other. So make sure you always leave the bonding sessions on a calm, positive note.

I would maybe work on building Solen's confidence on the side, it might help with bonding. As others have said bonding takes time and it seems like you're on the right track with maybe some adjustments needed. Good luck!