All the pouches / step bags presented are passable options.
I am a bit envious of guys that started with strap steps/ Stepps and never moved away from them.
Since you mentioned “ field sustainment pouches “; they are versatile and around 12”x 8” x4” . This size is closer to a haversack or sling bag.
They can be used alone, added to a larger pack or secured to a JX3 or stand.
Any mesh or rip stop dump pouch can sit in a larger pouch or any pack , and be removed from pouch to be clipped on saddle or belt for a climb.
It’s a workable option. I’ll do this sometimes from a gas mask / field pack (9”x 9” x 4” ) my ROS and bow hanger sit in the bottom and I just pull dump pouch on a s biner / steps out to climb. Or I ditch the steps pouch and sandwich stick(s) behind it.
I’d dwell more on weather, distance traveled and your platform / ROS as that will dictate the type of bag more than steps.
By that I mean a sling bag is great but lacks for over .5 Mile walks, platform or layers carry.
Pic is a field pouch on a small pack