error TS2749 dealing with const of list of readonly strings

I just forked remark-docx from github to make a small minor change and now when I rebuild it, I am getting some TS2749 errors:

Typescript issues with type casting

I am having some issues with types in TS. New at TS (Java last 25 years).

An object is created as empty and filled data later. How to tell TypeScript that the final result is not a partial without using type assertion?

If I have: