might want to create a separate thread in the developer forum
but the answer might be to use musl, but I haven't been down that road, trying to avoid self made images 
if you're using the official image, it'll resize the partition back to the original size, but if you're
creating your own images, you might put in a different size - MyBook Live Duo 21.02.2 / rootfs size - #10 by takimata
using two parallel openwrt installs/versions works too - Sysupgrade help for x86_64 - #14 by frollic
or just boot gparted from a flash drive after writing the image, and resize the rootfs partition,
or any other capable Linux dist.