now you have miui 7.4 xiaomi.eu 6.5.5. sviluppatore, its a xiaomi.eu ROM, then i think you already havetwrp und open bootloader. flashing xiaomi.eu Roms will not close the bootloader.
than gust go to ******** and built your new rom based on 8.0.5.0
just click the points in the rom builder you...