If you wish to avoid many-to-many, I would go with some kind of ‘proxy’ entity that stores favourites for your users.
Make a “favourite” entity, and set up a “1 to many” association from users to favourites, and a “1 to many” from projects to favourites. This makes sure a single user can associate to multiple favourites, and every project can associate to multiple favourites. That would circumvent the issue where only 1 user or 1 project can be favourited.
Of course, I do urge you to make some deletion behaviour so Favourite entities get removed whenever a project or user gets deleted. This could bloat your system due to the multiplicative behaviour between amount of users and projects in an active system.
I hope you can work with this solution in your application.