If you're struggling to fill it up, then just leave it empty. NAND flash works better when there's plenty of free space anyway.
The Mini 4 has a laminated display while the 2017 iPad 9.7 does not. I expect the extra cost of that display is why Apple only offers the Mini 4 in 128GB capacity to partly justify the price premium over the 2017 iPad. Out of warranty replacement cost for the Mini 4 is actually higher than the 2017 iPad.
Im embarrassed to ask but what is a laminated display and how is it better?