are you sure you can't expand root partition by adding other non-contiguous partitions with mtd-concat driver?
see here for mtd-concat https://github.com/openwrt/openwrt/blob/ec6293febc244d187e71a6e54f44920be679cde4/target/linux/generic/pending-5.4/497-mtd-mtdconcat-add-dt-driver-for-concat-devices.patch
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