There’s no API for phpBB to hook into.
You’ll have to constantly scrape the page, have the script go the links posted there, and scrape those pages. (And if there are more pages, they don’t load automatically, you have to ‘click’ the link for the next page, and scrape it.)
One thing you could do to not have to scrape constantly, is bookmark or subscribe to that topic, and triger the script from the email you get when someone replies.
And now with Ivelt strict counter-bot cloudflare verify human thing, I don’t think it is possible to scrape anything with a script.
Go with Claude, get what they said you’ll get, but with strict limits, or pay for Gemini, get very generous limits, but they’re secretly switching out the models for dumber ones.