Medication (sertraline).
i had PND after the birth of both of my children. Both were poor sleepers. With DC1 I thought it was just sleep deprivation. But after DC2 I was tired but happier and realised it was PND first time around. When I felt PND taking hold, and it was lockdown so I had limited options for easing it, I started sertraline and it was definitely the right decision. With low level depression for a long time, I wished I'd started it years ago.
Other than that, things that helped me were yoga, general exercise, more sleep (yes to sleep training, it's not essential but it definitely helped) and going back to work.