Here is some interesting stuff in the logs.
Sep 29 02:20:54 iMac-Pro Installer Progress[57]: Showing Progress UI
Sep 29 02:20:54 iMac-Pro OSInstaller[207]: Disabling system idle sleep.
Sep 29 02:20:54 iMac-Pro powerd[44]: Assertion Created. Type
reventUserIdleDisplaySleep Name:'OSInstaller' Id:0x500008026 Process:macOS Update Assistant [System: PrevIdle PrevDisp kDisp]
Sep 29 02:20:54 iMac-Pro OSInstaller[207]: Using product <OSProduct: 0x600002c8d380> at distance 5
Sep 29 02:20:54 iMac-Pro OSInstaller[207]: Setting shouldMigrate: 1 ((null))
Sep 29 02:20:54 iMac-Pro OSInstaller[207]: Will not update firmware because run-efi-updater is set to false
Sep 29 02:20:54 iMac-Pro OSInstaller[207]: SUSoftwareUpdateController: Service connection invalidated!
Sep 29 02:20:54 iMac-Pro Installer Progress[57]: IASSetCurrentInstallPhase: Current phase name set to "OS Installer 2"
Sep 29 02:20:54 iMac-Pro OSInstaller[207]: ROM Features from IODeviceTree:/rom returned: 0x00fd8ff53e
Sep 29 02:20:54 iMac-Pro OSInstaller[207]: ROM Features from IODeviceTree:/options returned: 0x00fd8ff53e
Sep 29 02:20:54 iMac-Pro OSInstaller[207]: ROM supports APFS
Sep 29 02:20:54 iMac-Pro OSInstaller[207]: ROM does not support large BaseSystems
Sep 29 02:20:54 iMac-Pro OSInstaller[207]: ------- Install Failed -------
Sep 29 02:20:54 iMac-Pro OSInstaller[207]: Operation: Verify firmware failed, Failure Reason: Error Domain=com.apple.osinstall Code=515 "An error occurred while verifying firmware." UserInfo={NSLocalizedDescription=An error occurred while verifying firmware.}
Sep 29 02:20:54 iMac-Pro OSInstaller[207]: Failed to find update volume. Attempting to flush to mutable product.