I don't think that this is possible to do with a marquee - it waits until the text is fully out of view before starting again. You may have to create something with CSS
I have thrown together a quick test of how it could be done with CSS. https://codepen.io/cbolson/pen/dygaBRL?editors=1100. This basically duplicates the text by using a data attribute and then displaying it in a pseudo element. The problem with my solution is that there are a few magic numbers in there. I have made them into custom properties so at least they are easy to adjust.