there are ways around that initial check by calling into the specific activity (UsbD2dMigrateFlowActivity) that does the migration, but the deeper issue with Android Switch is it needs permissions (manifest) you cannot set as user - that is why it’s shipped with the OS in official Android.
It would need the same kind of stub install that is used for Android Auto to work in custom ROMs.
If you’re into adb shell and your install can do adb root I could write a guide I think.
As said, the schema transition that is needed for WhatsApp iOS ChatStorage.sqlite to Android is well known. One can either do it by hand in sqlite or with any script language. There are also 3rd party tools. I can’t vouch for any of them, but WazzapMigrator looks like it can do it, it’s 10 bucks but still involves hoop jumping.