[my embolding] Soft, pliable stretchy, doesn't depend on the tanning method. Its the 'type' of leather, or where it came from on the animal. eg on cow and such animals [ buffalo, camel, horse] the part along the spine, the back, is thick and stiff, the belly part is stretchy and very pliable
I have 2mm belly leather that stretches like rubber [ok, exaggeration but almost] and 2mm back leather that is very stiff; both are veg tanned. I also have 'upholstery' leather which is chrome tanned, some of it can be stretched but some is much stiffer. The chrome tanned is pre-dyed and lacquered; its various shades of brown, I can use it like that or dye it black, but I can't tool it. Its also 1 to 1.8mm thick, maximum
You need to get veg tanned belly leather or upholstery leather or even 'clothing' leather - leather that is suitable for motorcycling 'leathers' is clothing leather which is stretchy