BTW, I tested KDE’s Clock flatpak, but the “Edit” button doesn’t seem to do anything is only used to delete an added location, and there’s not anything else that might enable seconds.
Correction: It does show an analog clock face with sweeping seconds hand, but the digital display doesn’t have a seconds counter.
Timer and Stopwatch both show seconds.
Demo of Clock: List of Apps that fit and function well [Post them here.]