The choice isn't whether to use one JavaScript library or Thymeleaf, that would be an artificial constraint you created, they are both different animals. Thymeleaf is for templating, JQuery is a client side library for making dynamic pages. It would make more sense to choose between templating libraries or choosing between JavaScript libraries, but both can go nicely together.
In general, your templating should not affect your JavaScript code, some people do template their Javascript code, but IMHO this is a terrible idea. Instead use Templating to replace tag attributes in HTML, and Javascript for making your website dynamic with whatever library you choose.
I'm not sure what you mean by
client-side approah instead of a server-side implementation
But what I see nowadays are 2 different approaches, creating Microservies VS Monolith web application. If you develop your website entirely in Thymeleaf you will not make it very reusable. Netflix, Amazon, Uber etc. all create services (sometimes called micro-services) which are reused throughout the organisation via a REST API.
If you only have a website implemented in Thymeleaf it will not be possible for another application (think mobile) to access any services created by your application, whereas if you create a REST API for your app then you can keep reusing your server back-end for when you need to make a mobile app, or another web app with similar requirements.
Of course if your services are not going to be re-used this might not matter to you and you might be very proficient in doing things a certain way albeit not being the most reusable, of course we are not always creating the next Amazon. But I still believe it is good idea to follow best practices, even if its just to get better at doing things a certain way.
