You can always unbrick your soft-bricked device using UART flash method (see wiki). But you will need UART-TTL adapter and TFTP server.
Run
hexdump /dev/mtd8 | head
on your device. If it starts with
0000000 0dd0 edfe
it should be safe to sysupgrade.
Otherwise you will need to use UART flash method to recover vendor firmware and then flash openwrt factory image from it. The downside is you will lose all your configuration.
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