I realize that perhaps my answer may not be entirely comparable to the question. But I want to share my experience. I was faced with the need to modify a dynamic page. All methods from this topic did not help me, but the following helped:

const observer = new MutationObserver((mutations) => {
    applyStylesToAllElements();
});

observer.observe(document.documentElement, {
    childList: true,
    subtree: true,
    attributes: true
});

function applyStylesToAllElements() {
    document.querySelectorAll('*').forEach(el => {
        el.style.userSelect = 'text'
    });
}