2. There hasn't been much time to develop the romance
There could possibly be a Season 4 if they can find a way to continue the story. It can be hard to come up with new plot ideas for a new season. It might be another year or two before we get another season, assuming Netflix has it pretty high on its priorities list.
The possibilities are there. I personally would like it if they did, but at the same time I find it unlikely
I think it might have been because Grougaloragran and his brother didn't have any relevance in the main plot of Season 3. Season 3 has a pretty linear story arc. It has moments of character development but it doesn't really go out of its way to include characters with little to no relevance to the main plot of the season.
Then again I haven't seen the full season yet, so I can't really say. (Too intense for me.)