Thanks, Yarrow, that helped.
The installer logs showed...
Skipping component "com.apple.CoreFP" (2.6.12-2.6.12-2006012000000-*) because the version 2.6.16-2.6.16-2006016000000-1 is already installed.
Skipping component "com.apple.DeviceLinkX" (5.0.0-264.0.0-264000000000000-*) because the version 5.0.0-264.0.0-266000000000000-552 is already installed.
Skipping component "com.apple.mobiledevice" (710.5.73-710.5.73-710005073000000-*) because the version 757.1.5-757.1.5-757001005000001-7 is already installed.
Skipping component "com.apple.MobileDevices" (1.0.0-710.5.73-710005073000000-*) because the version 1.0.0-757.1.5-757001005000001-7 is already installed.
Skipping component "com.apple.driver.AppleMobileDevice" (3.3.0-3.3.0-243000000000000-*) because the version 3.3.0-3.3.0-256000000000000-100 is already installed.
Skipping component "com.apple.SyncServices.AppleMobileDeviceHelper" (5.0.0-921.0.0-921001000000000-*) because the version 5.0.0-927.0.0-929000000000000-183 is already installed.
Skipping component "com.apple.AirTrafficHost" (351.6.0-351.6.0-351006000000000-*) because the version 448.0.0-448.0.0-448000000000000-110 is already installed.
Skipping component "com.apple.SyncServices.AppleMobileSync" (5.0.0-921.0.0-921001000000000-*) because the version 5.0.0-927.0.0-929000000000000-183 is already installed.
Skipping component "com.apple.macos.driver.AppleUSBEthernetHost" (7.0.0-7.0.0-113000000000000-*) because the version 7.0.0-7.0.0-122000000000000-305 is already installed.
...which is to be expected, so I cracked open the installer with Pacifict and replaced each of the components as well before reinstalling. Worked like a charm.