I used to be a road cyclist, and after moving to a coastal community where I was nearly run over 2X by sightseers intent on the ocean views, I switched to mountain biking. Locally, I mountain bike with flat pedals. I like the challenge of working harder on steep, technical ascents and descents, for our typical 1600-1900 ft climbs, at times multiple ascents. That said, for the Camino Portugues, I plan to switch out my flat pedals for SPDs with a flat side. My reasoning is that the daily grind of cycling will take a toll, so assisting the lift of my up pedal motion with the SPDs will help on climbs. The flat side is for when I don’t feel the need to clip in.
Then again, I’m starting from Lisbon, and I will eventually reach 2100 foot climbs along the way. Depending on where you are starting your Camino, you may not need SPDs at all, e.g., from Porto, which is practically flat terrain, having only about 500 ft elevation gain in a stretch.
Keep in mind that it is optimal for you to use the pedals that you are accustomed to. My husband has flat pedals, but he invariably opts for SPDs, but he rides only highly technical trails with lots of climbs and uphills.
On carrying your gear, whoa. Traditional panniers cannot be attached to mountain bike frames, cuz geometry. However, with increasing numbers of mountain bikers riding long distances (>30 miles on backcountry technical trails), a couple of companies have come out with fork attachments and dry bags to hold a small amount of gear. Top Peak is one manufacturer. Keep in mind that any weight attached to your forks will affect your front wheel maneuverability. Then again, I doubt as cyclists on the Camino Portugues, we will run into any rock gardens, gnarly tree roots, or jumps.
Buen camino!