if all the libraries you are using are as well...maybe.

You can write definition files for libraries. A lot of libraries already have such files: https://github.com/borisyankov/DefinitelyTyped

If a particular library doesn't have one, you can write it yourself. In that case theres no need to model the entire library - you can only model the subset of functions / methods that you use.

Or what if, hear me out, everyone just did that thing from the start?

I do tend to start all my new node projects in TypeScript, yes.

With TS do I have to have a spec for every third party code or can I import something and use it as-is, but not get the benefits of TS on that part of the code?

You can do both.

Seems like a silly conundrum to be in.