Proroute H820 unused flash partition

My router has a large unused partition between the factory and firmware partitions:

root@OpenWrt:/# cat /proc/mtd
dev:    size   erasesize  name
mtd0: 00030000 00001000 "u-boot"
mtd1: 00020000 00001000 "u-boot-env"
mtd2: 00010000 00001000 "factory"
mtd3: 001a0000 00001000 "user_fs"
mtd4: 00600000 00001000 "firmware"
mtd5: 0016ee5b 00001000 "kernel"
mtd6: 004911a5 00001000 "rootfs"
mtd7: 0021f000 00001000 "rootfs_data"

Is there something I can configure to add it to rootfs_data?

I now know some things about the version of uboot on the device but sadly the disassembler wasn't able to find a reference to a relevant string I could have used to figure out where to hexedit the default load address.

The default load address is 0x20000 and I want it to be 0x6000.

I'll be trying again but I have a question, does anyone know if it would still boot if I changed some bytes in the uboot code stored in the flash?

I have some old equipment I can use to replace the flash chip but I'm waiting on some things to arrive before I can do that.