This code piece can be used in Tempermonkey scripts. Execute it on page init and it will wait for dom until target element shows up. Then you can get the elements you requested and work with it.

PromisedGetElements.js

// need piority: document-start
function waitForElm(selector_list) {
    let result_list = new Array(selector_list.length).fill(null)

    // request all elements in selector, if all elements are set, then return true.
    function collect_selection() {
        selector_list.forEach((value, index) => {
            if (!result_list[index]) {
                let search_result = document.querySelector(value);
                if (search_result) result_list[index] = search_result
            }
        })
    }

    function check_list_complete() {
        return !result_list.some(item => item === null);
    }

    return new Promise(resolve => {
        collect_selection()
        if (!check_list_complete()) {
            const observer = new MutationObserver(() => {
                collect_selection()
                if (check_list_complete()) {
                    resolve(result_list);
                    observer.disconnect();
                }
            });

            observer.observe(document.body, {
                childList: true,
                subtree: true
            });
        } else {
            resolve(result_list)
        }
    });
}

// example for bilibili.com
waitForElm(['.bpx-player-video-wrap video', '.bpx-player-dm-root', '.bpx-player-video-inputbar']).then(elements => {
    let [bilibili_video_element, toolbox_element, toolbox_danmu_input_box_element] = elements;
    console.log(bilibili_video_element, toolbox_element, toolbox_danmu_input_box_element);
})