Hi there,
Total Block Time, is a measurement of how long a user has to wait before they can make an interaction with a page, such as tapping a hamburger icon to open the menu.
If you use any optimizations that defer or delay the loading or scripts then you will possibly increase TBT.
You should ask WP Rocket for their input on what settings should be disabled or changed to minimise.
DOM size, for every piece of content, text, image, button, comment etc. that is added to a page adds an element to the DOM. In simple terms the more content the bigger the DOM size. It only becomes an issue if their are a lot of scripts or style / layout changes on the page as it can tie up the browser. I would ignore it its just a diagnostic not an error.