I'd you're asking it to write in "plain English", that's your problem. Plain English is English that is easy to understand. The text ChatGPT writes is very easy

I'd you're asking it to write in "plain English", that's your problem. Plain English is English that is easy to understand. The text ChatGPT writes is very easy to understand, if a bit too verbose. Add to your custom instructions something like "Always be concise and direct. Avoid filler words as much as possible".