I am reading all this with a newly-inspired interest.  I recently acquired a full-sus bike that I intend to go trail centering on, as well as seeing how well it deals with local byways and the Ridgeway.  I am a full-on SPD convert and use them exclusively on my hardtail and gravel bike.  There’s no way I would change this.  However the new bike, currently pedal-less, has made me think about what to do.  Flats are de riguer for this sort of bike and technical use, and I am willing to give it a go.  I wondered about swapping pedals dependent on use - SPDs for local use and flats for trail centre, but I can see this getting tedious if I flip-flop between the two.  I’ve never seen pedal that combines SPD with a proper flat platform.
I guess try-it-and-see is the best approach!