I suggest you try a few different / older Netinstall versions, that's how I was able to restore RouterOS on my RB750Gr3. Neither latest v7 nor latest v6 Netinstall worked for me, however I used 6.44.5 which DID work.
You will know it works when you see the progress bar while formatting the flash as well as installing the OS - make sure you get the matching Netinstall and RouterOS package versions