I had the same issue going from 18.06.4 to .5 but back then sysupgrade succeeded. Previous issue and resolution detailed here.
First I tried to flash via Luci. Yes, I did confirm the sha256 checksum. After flashing via Luci I was still on 19.07.0.
Next I tried using sysupgrade as detailed in Upgrading OpenWrt firmware via LuCI and CLI. Again after the router rebooted it was still on 19.07.0. I even tried updating all packages, which worked in version 18 but no go.
Finally I tried mtd. I'm not sure I know the correct target for the command. The upgrade doc specifies the command as
mtd -r write /tmp/firmware_image.bin firmware
So I used the command
mtd -r write /tmp/openwrt-19.07.2-bcm53xx-netgear-r8000-squashfs.chk firmware but I'm not sure the target "firmware" is correct. Any ideas?