Managed to solve it (with a minor waiver on my end) by adding a new doxygen group:

/// @addtogroup SignalTypes
/// @{

/**
 * @brief Template struct representing a generic signal.
 * @tparam SIGNAL_TYPE The data type of the signal value (e.g., double, float, uint8_t, etc.).
 */
template <typename SIGNAL_TYPE> struct Signal
{
   bool valid = false;   ///< Indicates the signal is valid.
   SIGNAL_TYPE value = 0 ///< The signal's value

   /**
   * @brief Equality operator for Signal.
   * @param other The other Signal to compare against.
   * @return If both signals are available, compares their values.
   * If not available, checks if the other is also not available.
   */
   bool operator==(const Signal<SIGNAL_TYPE>& other) const
   {
     return (available ? other.available && value == other.value : !other.available);
   }
};

/// Type alias for double-precision floating point signals.
using DoubleSignal = Signal<double>;

/// @}

And using the output file group__SignalTypes instead of the current Interface_8h file I used previously (this is the small waiver :) )

Why is this fine by me?

Since now the Typedef types aren't in a single namespace page the user can't find anywhere and has a LOT of other items in.