The recent transformation of the smart grid has led to a complex and heterogeneous cyber-physical system (CPS). The modernizing electric power infrastructure is equipped with multiple systems for sensing, communicating, controlling, and processing an extensive volume of data, for which a platform or a testbed mapping various components of a smart grid is extremely useful for R&D purposes. To this end, a testbed featuring one simulator will not fully characterize the functionalities of such a complex infrastructure. A Co-simulation testbed that federates loosely coupled standalone sub-simulators is appropriate and will accurately represent a Smart Grid. This paper will investigate the process and pitfalls observed in the re-development of ASGARDS-H at Concordia University, Montréal, a co-simulator based on the Hierarchical Engine for Large-scale Infrastructure Co-Simulation (HELICS) for 5G-based smart grid security. The paper will reveal the designs and modifications of the testbed under a microservice architecture via containerization. New tools and code refactorings are proposed and discussed to decouple the simulation logic from the time and value synchronization of the co-simulation. Beyond the final platform that offers improved simulation capacities and functionalities from ASGARDS-H capabilities, the paper also aims to share lessons learned and notable pitfalls that can help smart grid security researchers more effectively and efficiently develop flexible, reliable, and scalable co-simulation testbeds.