Hi darkarchon,
I don’t own a Tuft & Needle mattress but I can make a few comments and hopefully others that see your post and own the Tuft & Needle will share their thoughts as well. How your platform bed compared to a bunkie board would depend on the type of bunkie board you were considering and the specifics of the support surface on your platform bed. If a bunkie board has a solid surface it would be more rigid and evenly “supportive” than many foundations or platform beds that have wider gaps between the slats but it would also provide less airflow under the mattress (see post #10 here). I would tend to avoid a flexible box spring with a foam mattress unless there was a specific and compelling reason to use it because it can affect the feel and performance of a foam mattress which usually does best with a rigid non flexing and evenly supportive surface underneath it.
There are some slatted bunkie boards listed in post #4 here and posts #10 - #12 here may also be helpful if there are wider gaps between the slats on your platform bed than you would prefer.
Phoenix