I’ve been riding flat pedals exclusively for at least 5 years now after a good 20yrs on clipless. While I’m primarily a mountain biker I do a lot of road and gravel as well.
My reasons were mostly for bike handling - I realized spd’s had introduced a lot of bad habits for me as they let you get away with poor on bike positioning, pulling up on cleats when bunny hopping and worse pedalling mechanics. In flats, if you do these things you get immediate feedback when your feet come off. That’s obviously a drawback as well for flats.
I also suffered from foot pain & numb feet with spd’s and that went away completely for me with the flats which is why I now use them on the road and gravel as well full time. Sore uncomfortable feet really detract from the joy of riding.
Personally, I use Pedalling Innovations flat pedals on all my bikes. They have a longer platform and give your foot more support especially if you choose to use a more mid-foot position, which I personally prefer. If you go that route, you’ll likely need to drop your saddle a bit as well as scooch it forward a bit. I have used smaller flats and I find my feet get more tired than with the larger platform. Nothing like the discomfort of the spd’s but I missed the bigger platform for sure.
For 5 Tens, get one of their lighter models. I use the Freerider pros but I’d love to try some of the newer ones for road/gravel like the Trailcross. Shoes like the Impacts are boat anchors. I’d only use them for DH.
Quick mention of the negatives of flat pedals: there is a learning curve to using them and your feet will come off at times you don’t want them to (happens less and less as you improve and far less likely for road/gravel); your foot doesn’t go to exactly the same spot every time and adjusting position can be a bit of a pain as the sticky shoes really do work (this has never led to any riding induced pain or repetitive use injury - flats let your body adjust as you need to and I think makes it less of a risk than with spd’s which lock you into a pretty fixed position); if your feet come off and your shins hit the pedals, you’ll have some scars for life (I wear shin pads now for almost every MTB even though my feet rarely come off now); heavier; probably slight efficiency loss (I don’t believe it’s much though and definitely not more than what you lose from discomfort and getting off your bike).
Last comment is wrt pedalling technique - use all the TR pedalling drills. The only one I couldn’t do effectively was true one leg pedalling. For those I kept both feet on the pedals and let one leg just go limp and go along for the ride. The drills will show you that you can still be active all the way through the pedalling stroke even the 9-12 a clock portion. While your feet aren’t pulling on the pedal, you are getting your foot out of the way so the other foot isn’t pushing against your trailing foot. Even for spd, that is the main point of the exercise as you don’t generate any real power pulling on your cleats at that position.
Good luck - hope you enjoy the flat pedals and it leads to pain free riding.