I understand not wanting to have to replace the iPads, but I’d get new tablets before I downgraded to such an old firmware version. You can actually get an iPad 9th gen for $200 right now on Amazon or if that is too much too spend, the Amazon tablets are all sub $100 right now too.
EDIT – it seems that perhaps the 9th Gen iPads are currently OOS, but they have hit this price pretty often over the last few months (I’ve bought two myself at different times), so I’m sure they will be back at this price even after Black Friday at some point.