That's a solid default. I actually set my RSS polling interval to 1 hour, most sources I follow don't update frequently enough to justify anything shorter. Every 10 minutes works too, but you might end up burning cycles on unchanged feeds.
Let me give you a simple detection algorithm. Apply OCR to the screenshot because they often use logos. Also, parse the text from the HTML and compare it to the URL. You can catch a lot of spam this way.You can also examine many parameters in the js html code.
That's why I'm developing a system that only allows messaging with authorized senders using email addresses, chat addresses, and phone addresses, and a tool that feeds anonymized information into an LLM API, retrieves the output, reverses the anonymization, and responds to the sender.