Similarly a sidemount-style butt pouch would work for all the secondary stuff.
I'm looking at these butt pouches and my backplate and finding it hard to image clipping double-enders through the little holes in the bottom of my backplate, behind me while underwater. Maybe it would be easier to leave the double ender attached to the backplate and clip/unclip it to the bag? More shockingly, I can't find any videos or anyone talking how to easily access a butt pouch.This is my go to for warm water, even with thigh pockets on my wetsuit.
EDIT: Oh, sidemount setups have butt plate with large, easy to clip attachment points for the tanks. I assume the pouches are made to attach to those, not the little holes on the plate. Seems like the pouches would be a pain to use without that.