When I was using emacs I did it just using the internal stuff except for avy. You can get very far without extensions, though you may need lsp mode if you do frontend work.
Indeed you can get quite far with only the built-in modes. One pain point though is the lack of many major modes, for example there’s no built-in markdown-mode, json-mode, julia-mode, or typst-mode, to mention a few.
I recently did try to run Emacs without any packages for a while. To my surprise, AUCTeX was not one of the packages I missed, the built-in latex-mode works well. I did miss CDLaTeX though.
