"Writing betterer than before"- I admit I misread it first as "better" :D
If you are writing for Steemit grammar mistake may be mistaken for a foreign speaker with bad grammar.

"The Writing Secret I Killed For"- this one is catchy but very comical has to fit the context.

Definitely no:
"secret of writing"
"the writing secret"
Sounds too much like "princess diaries"
"secret writing diaries" :P
Gl!