I managed to get around the above limitation on FHIRKit Client module by expanding Node.JS trust store with company Root CA, using the environment variable NODE_EXTRA_CA_CERTS as explained here.

set NODE_EXTRA_CA_CERTS=C:\\Projects\\FHIR_Client\\RootCA.pem

This way, I managed to avoided disabling certificate altogether on Node.JS with:

set NODE_TLS_REJECT_UNAUTHORIZED=0