Thanks for following up, your doc and links RbnJrg!
SumatraPDF works under Linux with Wine, it's even a snap package:
https://snapcraft.io/sumatrapdf
It's the only one I found that automatically reload the epub file when I save in Sigil so it can sit there while I work on Sigil.
I forgot to say, I'm working on epub 2 now, just to check what is working for old e-readers.
I expect my result to work in epub3