What SPD pedals are you using? Before I went SPD-SL’s I did 100s of miles with SPD’s and a touring pedals ( PD-A600 (shimano.com)). The pedals were light and had a decent cage. They are single sided though so I’ve went recently with double sided Richie Comp Trail pedals for my gravel bike ( Ritchey Comp Trail / Mountain Bike Pedals (ritcheylogic.com) ). I’m still to do substantial miles on then though.
Edit the only problem I find with flats is when you stop if you push off with one dominant foot the pedals may not be in the position to do so, its easy to raise that pedal/foot with SPD’s and the like (I’m hesitant to call them clipless 
)