Starting out on clipless with road SPDs isn’t that easy but MTB SPDs are far easier to use. PD-M520s with light cleat tension could be good.
It’s not like trying to get a foot out when you’ve just hit an off camber root that’s trying to take you down, plenty of time to think about it on the road.
I’d say you have a lot more chance to react or recover it on the MTB off-road. On the road bike you can go from mid corner to sliding on the tarmac in an instant and you have pretty much zero chance to correct it. So you may as well clip in, flats won’t save you.
I almost highsided myself into space when I had my horizon comps on the cross bike,
Road bikes with MTB flats have pretty low lean angle or pedal clearances, it’s a valid point.