Shiny chances are 1/8192 and hardly doubt that summer chances would rise above 1/1000 (my guess is 1/500).
Just keep on grinding and you will find what you are looking for eventually mate!
It is about RNG and nothing else I have been farming Scythers 10+ hours a day since start and got none.
But..
Spawn rate is still fine.
I know people that have got several summer forms.