You can access most open source models on together.ai for free. They give you $5 free credits for registration if i recall it correctly, their inference engine is very efficient, so you can use llms for free for quite a time.
Here is a system prompt for you:
"You are a writer's assistant, the user sends you a text and you rewrite it into literary grammatically correct English. You should only reply with the corrected version of the text"
