Send all mails in a ProtonMail search query to any folder of your choosing

folderEmails.js

function waitForElm(selector) {
    return new Promise(resolve => {
        if (document.querySelector(selector)) {
            return resolve(document.querySelector(selector));
        }

        const observer = new MutationObserver(mutations => {
            if ((document.querySelector(selector) !== undefined) && 
                (document.querySelector(selector) !== null)) {
                resolve(document.querySelector(selector));
                observer.disconnect();
            }
        });

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

const folderPage = async function(folder) {
    (await waitForElm(".item-container:not(.item-is-loading)"));    // wait for messages to show up
    (await waitForElm('#idSelectAll')).click();
    (await waitForElm('[data-testid="toolbar:moveto"]')).click(); 
    (await waitForElm('[data-testid="folder-dropdown:folder-'+folder+'"]')).click(); 
}

const folderEmails = async function (folder) {
    await folderPage(folder);
    var nextpage = await waitForElm('[data-testid="pagination-row:go-to-next-page"]');

    while (!nextpage.disabled) {
        nextpage.click();
        await folderPage(folder);   
        nextpage = await waitForElm('[data-testid="pagination-row:go-to-next-page"]')
    }
};

await folderEmails("Trash");

This is great. Can you figure out how to delete all my labels, too? :)

How many labels do you have? I don't have that many so I would probably just do it by hand.
But the key to this script is using waitForElm. If you want to delete all your labels you can just go the folders folder, keep calling waitForElm to get the next label, and delete it until all your labels are gone.