The platform SPDs don’t really give much more support to the shoe unless your shoe is flexible enough to be making contact with the platform, in which case your overly flexible shoe is probably what’s causing the issue in the first place…
i have no issues with 200-300km road rides using my stiff shoes, but my bendy Spesh Tahoe’s cause some problems on >100km on road, off-road I tend not to do more than 100km at a time.
there is something to be said for flats on long off-road or touring rides, decent shoes and decent flats can be just as if not more comfy than SPDs, depends what you’re used to and the type of ride, I flip between the two and mostly only use SPDs off-road when I’m racing XC or on a long mixed surface kind of ride.
Stiffer shoes or stick with your flats would be my suggestion.