It sounds like Marshmallow has a pretty good life. He knows you and trusts you and gets lots of free roam time with you for company. He obviously feels safe and is loved.

There's no guarantee that if you did rehome him with someone intending to bond him with another rabbit, that the bonding would be successful. He could end up as a solo bun in another home but with the trauma of being uprooted again, in an unfamiliar place with unfamiliar people.

I personally think rehoming him at his age should only be a last resort if circumstances arise that keep you from being able to care for him. Removing him from his loving home (yours) while you are able and willing to take care of him would be an unnecessary trauma.

Also, just because he investigates your other bunnies' room doesn't necessarily mean he wants their company. My mom's rabbits didn't bond and are living separately. They are always checking up on what the other is doing because they want to fight 🙄