How to clean install of MacOS on 2018 MBP
According to everymac.com, your 2018 Mac should be able to run macOS 10.13.6. You could try using the 2014 Mac to download High Sierra from this website.  This download should be the latest High Sierra macOS 10.13.6. You can the use the instructions from this website to transfer High Sierra to a flash drive. After booting from the flash drive, use the Disk Utility to erase and APFS format the internal drive. Finally, try installing High Sierra. If successful, then you can try upgrading to Catalina.