The firmware you're running now is not from the official OpenWrt project. So it is not a surprise that you are getting that error.
I don't know if it is safe to force that flash, but if you do decide to go that route, you must not keep settings as it will almost certainly soft-brick the device.
The recommended path is usually to install OpenWrt from the stock firmware. That said, you may want to see about reverting to stock first, and then following the directions to install OpenWrt directly from there.
Really? It's from robimarko..
Actually, I don't remember if it's from robimarko or that I've directly downloaded from https://firmware-selector.openwrt.org/
LuCI is only different because I've installed Argon as the default theme.
I wouldn't keep. I usually force reset in every sysupgrade.
But I don't want to face a soft brick and make a simple upgrade to a TFTP challenge
Yeah.... that's what I was thinking....
I don't know if @robimarko will see this, but if he could also tell me if that's the only safe path to take...
The problem of going this route is that I'm gonna need 2 routers.