What can be done to improve mobile page load performance after you optimized all the assets (css,js,images)?

We have WordPress website with a joing page. (https://www.firstclassflyer.com/join/). Right now, our mobile pagespeed score is around 50, and the first contentfull paint on mobile is around 3s, which is not that bad. But we want to take a step further and cut it down by 50% if possible. We're using some plugins (page builder and form builder) and we did what we could to optimize assets: lazy loading / webp images, lazy load some css / javascript files, etc. Is there anything else we could do to improve the first contentfull paint? If possible, we would like to keep the current layout, and still use the page builder and the form builder. But I'm open to any kind of tips or…