Your sysupgrade -F -n with factory should work. It looks ok.
Some ideas for you:
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use luci-app-advanced-reboot to see what it thinks to be the contents of the other partition. I assume that it contains the OpenWrt you flashed, but that it doesn't boot.
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Note: Changing just boot_part is not enough. You would need to change also "bootcmd". ( But those are automatically changed by the sysupgrade, so it should be unnecessary )
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One possible reason for the boot failure is a bad block in the kernel area in the nonfunctional partition. That would cause factory image to get flashed wrongly, but a normal sysupgrade would work. (See
Increasing mamba and venom kernel partition to 6MB - #122 by bmork for discussion).
You might overcome that if you first sysupgrade normally again with the newest 19.07 sysupgrade image, so that you could sysupgrade normally and the boot gets switched to the other (now nonfunctional) partition. Then you could try force flashing again the 21.02.3 factory image (to the current partition) in hope of this being intact. (Sysupgrade is always round-robin in your router, so the next flash goes to the other partition.)