Just a few small thoughts …
Given that the eMMC drive isn’t that fast, I would like to see the speed of the uSD card increased to match that of the eMMC drive. Is this possible? I don’t know.
Then one could contemplate unifying the storage across both i.e. the root file system on a single logical storage device spread across multiple (two) physical storage devices. The kind of thing you get with LVM. Is this possible? I don’t know. (Of course if you do this then you completely give up either removing the uSD card or having a set of uSD cards to flip in and out. So it would be just one way of working that a customer would choose, if giving priority to available space over other things.)