Well, I remember I also once had problems with VirtualBox so I decided to switch to the binary package (virtualbox-bin). I also have just the virtualbox-modules installed, both in version 5.2.22 from portage. It might be worth a try, so if you uninstall everything related to Virtualbox and just emerge virtualbox-bin and virtualbox-modules I hope you will succeed. As a sidenote it is the case that in virtualbox-bin the executables changed from lowercase to camelcase. The additions can be installed manually from the UI, as said no need to do this with portage.
However, as I do not have a AMD Ryzen system the problem might be something completely different...