Answer
Use plsql_rest_sender_api.call_rest_endpoint2 instead — it accepts a CLOB body together with URL parameters built using Documents rather than Records, which is what actually works for this combination (the base call_rest_endpoint documentation's example for URL parameters doesn't hold up in practice). Switching to the _2 variant of the call resolves the argument-mismatch error while still letting you pass URL parameters dynamically rather than hardcoding a separate Routing Address per endpoint.