It seems like the whole point with a combined web/REST GUI is to make the backend part of the system smoother, so it does not have any references to web pages. It can then instead have references to the REST URI's instead and the presentation client layer can be easily changed to something else, like from Angular to React. We also get an interface without any visual stuff in our backend system and we can call this from other backend apps. Wouldn't this be impossible if there was Thymeleaf pages on the backend system? It seems like Thymeleaf would ruin the whole point of a combined web/REST solution.
You can choose to let your client load the data with AJAX calls to your REST services, but sometimes you just want to build parts of your web-page at the server-side, especially when they don't change after loading.