Use DDU (Display Driver Uninstaller) with the Internet disconnected and deleting the AMD Driver Installation folder at C:\AMD.
When it reboots back up install the appropriate Boot Camp driver.
Hopefully, DDU will remove all traces of the previous AMD driver which might be preventing you from installing any other AMD Driver.
Found this eGPU Tech site on how to setup Bootcamp eGPU. It shows you how to use DDU in Boot Camp environment.
Plus possibly it might help you install your AMD driver from some information about how to setup the eGPU: https://egpu.io/boot-camp-egpu-setup-guide/
Windows 10 can detect new graphics cards and install the drivers automatically. However the drivers are not up-to-date and sometimes interfere with setting up an external GPU. We recommend using DDU to uninstall the Radeon Pro drivers that came with Apple Boot Camp drivers (part of Boot Camp Assistant in macOS). DDU can also disable Windows automated graphics drivers installation so that you have full control of which drivers version to use. If you have a Mac with Radeon dGPU and plan on using a Radeon eGPU, the modified drivers from bootcampdrivers.com is the best approach to use both dGPU and eGPU concurrently.