There is one an effective way to solve this problem. It can be not easy -

put all root (besides boot) into 32GB flash and use fast 256MB space as a cache for disk blocks. Initially it would take some additional time and battery to replicate blocks into new 256MB cache but later it would give a fast access to frequently used data with virtually unlimited space.

It is possible just to extend kernel cache interface to include 256MB into it. But to get the result permanent via next reboot it may requires some additional development in FS.