Whatever language you chose you might want to also look at the htmx JS library. It lets you write in your html snippets better interactivity without actually needing to write JS. It basically lets you do things like when you click on an element, it can make a request to your server and replace some other element with the contents your server responds with - all with attributes on HTML tags instead of writing JS. This lets you keep all the state on the backend and lets you write more backend logic without only relying on full page refreshes to update small sections of the page.

For a backend language I would use rust as that is what I am most familiar with now and enjoy using the most. Most languages are adequate at serving backend code though so it is hard to go wrong with anything that you enjoy using. Though with rust I tend to find I have fewer issues when I deploy something as appose to other languages which can cause all sorts of runtime errors as they let you ignore the error paths by default.