Batch convert odf to png saving transparency

Hi all,
how are you?

I would like to convert a lot of odf files in png files saving transparency (e.g, the white background color becomes transparent). Seemingly it is the default when I use the GUI, but when I run:

soffice --headless --convert-to-png myfile.odg

I got a poor quality png file with a white background.
Could someone help me, please?