Welcome back @sakuraime . You were very close to having it.
You can get the whole text file with lookup, but you need to specify delimiters which do not exist in the entire file. For this I recommend using control characters such as bell. The results may be returned in a single-element array, but that shouldn't be a problem.
Depending upon the source, you may also be able to fetch the content with a Web activity. This approach is more work, as you need to do the authentication and headers, etc.
I have tried to find the limits of Set Variable. I don't recall the exact limit, but casual use did not run into it. You might run into a limit if you tried to store a gigabyte file or the text of a full-length novel.