I have bodged around the cross compile problem with a highly non-replicable local hack and how have hit what feels like a "final boss" error message:
-Djava.ext.dirs=C:\Users\Andi\AppData\Local\Android\Sdk\build-tools\33.0.1\lib is not supported. Use -classpath instead.
At no point does -Djava.ext.dirs appear in the invocation line that produces this error. In addition, the application I am building is written in C