The integration of EtherCAT and TSN has been proposed to enhance performance of EtherCAT networks in industrial automation. EtherCAT over TSN transforms a traditional EtherCAT ring into multiple shorter rings interconnected via TSN switches, enabling concurrent data transmission across segments and reducing cycle time. However, the use of multiple segments introduces contention for the master in the return path, potentially leading to scheduling failures and an increase in cycle time. We study the impact of network topology, i.e., the number of segments and slave node distribution, on schedulability, and formulate the topology optimization problem for the converged network based on schedulability analysis. We evaluate our methodology and optimal solution using SMT and Integer Programming (LIP) solvers, respectively. Numerical results demonstrate the effectiveness of our method, and our solution outperforms baselines.