Open the DevTools Console on your watchlist page and run this code.

Clear TMDB Watchlist.js

document.querySelectorAll(`.details .title a[href^="/movie"], .details .title a[href^="/tv"]`).forEach(async (titleElement) => {
    const titleType = titleElement.href.split("/")[3]
    const titleId = titleElement.href.split("/")[4]

    const rawres = await fetch(`https://www.themoviedb.org/${titleType}/${titleId}/toggle-list-item?translate=false`, {
        method: "PUT",
        headers: {
            "Content-Type": "application/x-www-form-urlencoded; charset=UTF-8",
        },
        body: "type=watchlist",
    })

    const res = await rawres.json()

    console.log(res.message)
})