I just watched the movie again not long ago. In the movie it is specified that Ra's
PEOPLE were dying, until Ra found Earth.
Here's the clip.
Ra is not the only survivor of his species in the movie, he's just the only one we see. This is not a retcon. Likely other members of his race would have been encountered in the sequels.
I think the TV show producers took the "like a parasite" line a little literally when making the TV show versions, but there is nothing in the movie to contradict the Goa'ulds natural appearance in the show either. There's no reason the Alien form we see has to be the original Goa'uld form - it's is just a visual representations of Daniel telling the story (Daniel doesn't know what the Goa'uld look like) and even if the flash is true, there's no reason not to suspect that's not just another host body, one that's failing.
There were two more movies planned that Emmerich and Devlin kept attempting to start once in awhile, most recently around the time of the Independence Day sequel. However, with that bombing, the Stargate sequels were completely abandoned. Emmerich confirmed this in the last week doing press for Moonfall.