I have a project (Maranatha2) that has a few image files and 365 daily text files (0101.txt thru 1231.txt). I am guessing that when I try to save this project as a new name that it is failing due to the 365+ files. None of the text files are very big.
I will try deleting the daily files to make the copy then reload the files for the original project. Wish me luck!
Q
If this text files aren't that big, consider storing the data for each textfile in a list or in the tinydb...
Yea, list might have worked but with various punctuation in the text, I am not sure how well I could get that to work. When I deleted the files down to 60 text files and I was able to do the Save Project As. Having the 365+ files ONLY failed when copying the project. I was able to get the original project with all those files working fine. 
Since deleting files to copy the project worked, I am good to go.
Q
Remove some files from the assets, build the app and add (reinsert) the missing files:
I have a desktop program that can take lines of text and produce an App Inventor Blocks List, meaning no need to type it all in block-by-block. I don't know what the length limit of a text item is though....... If you want to send the files to me I can see if they will be good to go as a Blocks List.
TinyDB is a good choice for predefined data too, you can load it up via a Procedure that reads the files and saves them as text data. It would be very nice if developers could pre-define TinyDb for this sort of purpose. It has been discussed, having been proposed by TimAI2.
If the number of files is a limitation but the files are small, loading them into a spreadsheet with MMDD in column 1 and the corresponding text in column 2 would allow you to export the sheet in CSV format, which could be loaded from a single file into a table, for use with the lookup in pairs list block.