I see that you’ve asked this exact question in the place it’s most likely to be answered.
The KDE forum: https://discuss.kde.org/t/how-to-show-seconds-on-sddm-and-lock-screen-in-kde-plasma-6/11778
Based on several forums, it seems the method you tried is the only publicly known method at the moment. This means your best chance of getting a solution is from someone who works with KDE more directly.