Step executed on init phase (only once and reused)

You could try putting the file contents into the run context. In the test case Setup script put this:

// this gets the appropriate context depending if we are in a load test or not
def ctx = context.LoadTestContext ?: context 

if (!ctx['myFileContents']) {
 // if the file has not already been loaded, do so
 ctx['myFileContents'] = new File('c:/test/test.txt').text
}

This loads the file when running the test case normally. Now in the load test Setup script put this:

context['myFileContents'] = new File('c:/test/test.txt').text

Then in your requests, refer to the file contents this way:

${=(context.LoadTestContext ?: context)['myFileContents']}

Each thread will still have to construct the request with the full text but this avoids loading the file load on every execution and having multiple copies of a property with the same contents.