Breadth-first search with ack or grep
I don't know ack but with tools such as grep I typically use:
( find . -maxdepth 1 -print ; find . -mindepth 2 -print ) | xargs -n 50 -exec grep TXT
The part between ( and ) ensures that first the files at level 1 are listed and after that those at 2 and deeper (you can vary). xargs feeds the file names per 50 to grep.
Of course it depends on which variant of find is available on your platform. If running something from 30 years ago, you will need to use something like sorting on the number of forward slashes.