@Damien Mccormack As you have surely discovered, a board can only belong to a single Space.
Other Options
1. You can embed a board I to other boards. And rather than have long the embedded board open to the default start view of the board, or the last location the user was on the board, you can chose from a list of starting points, e.g., frames or Slides - more on embedding here.
2. Synced copies. If you are on the Business or Enterprise plan, you could put content related to several projects into widgets that can have synced copies (AFAIK that includes Frames, Docs, Tables, Diagrams, and Slides) and then place copies of those widgets on as many other boards as required. When you update the original board's content, all copies of the widget are also updated. More on this here.