Although still requiring software understanding to implement, I've just remembered seeing a reference to mtd-concat in this thread which could make it easier than I earlier thought. Being able to use mtd-concat would allow you to join the existing root filesystem/overlay space to the free space at the end of the larger flash. Provided the device boots from the larger flash, the changes to do this are all in the OpenWrt environment and you would likely get away without needing to change the bootloader at all.
There are quite a few existing examples of devices using this functionality (I count at least 20 in the ramips target alone for OpenWrt 24.10). My suspicion is that after copying the contents of the old flash to the new flash you would just need to define a new partition in the device tree for the unused space and then add the reference combining the new and existing partitions to the device tree. In theory, this would only require building a new device tree binary. While it isn't that hard to extract a DTB (device tree binary) from an image using tools like binwalk, and decompile/compile DTBs to/from source (DTS) using dtc, there aren't any convenient tools to replace the DTB in an image with a modified DTB while preserving the image structure so building the full image from source becomes a necessity.
At least Netgear bootloaders usually have reliable and relatively easily used ways to flash back factory images...
I've never tried to do this myself so the above is hypothetical - be careful and do your own research!