Connected and automated driving introduces a myriad of new V2X services, such as cooperative perception and maneuver coordination, that significantly increase the channel load and require multi-channel V2X operation. Policies for Multi-Channel Operation (MCO) and congestion control are therefore essential for simultaneously supporting multiple V2X services across several channels. In this context, this paper presents the design, implementation, and extensive validation of a Facilities layer V2X congestion control solution for multi-channel operation integrated into an ETSI-compliant Cooperative Intelligent Transportation Systems (C-ITS) protocol stack. Our approach dynamically adapts transmission parameters based on real-time channel conditions and the priorities and requirements of the V2X services operating in a C-ITS station. By employing a Traffic-Class based proportional fairness strategy, the solution allocates available communication resources among multiple V2X services, effectively responding to varying channel loads in real time. Scalable experimental results in a virtualized environment demonstrate that our solution meets ETSI Release 2 requirements while bridging the gap between simulation-based evaluations and real-world testing, accounting for hardware limitations and processing delays. This work lays a robust foundation for scalable and congestion-aware C-ITS testing and validation prior to real world deployments. This paper makes our code publicly available so that other researchers can replicate our study and further explore MCO solutions for V2X communications.