Rewriting vanilla JS to Typescript isn't that big of a deal though. It would be more like building a house on a budget, then adding some fancy paint, furniture and alarm system.

In my experience, if you spend a few weeks in the "exploratory" phase writing ES6, rewriting to TS won't take more than one or two days.

Nowadays I'm a lot better at Typescript and will use it from the get go, but for someone who is less skilled in it, it might much faster to produce working code first and add types later.