Summarization for survey open end questions

Hi I’m working in HR and would like to summarize key topics in employee questionnaire.
Let’s say there is a question " what effort is required to improve onboarding experience for new hire?" . There are 6000 replies and each answers ranges from 3 words to 200 words. Because this is not a single document but multiple documents, I consolidated all comments into single document and applied various summarization model(T5, Bart…). However, the summary result does not look correct. Can I ask if there is a model/method which reads multiple documents and provide a summary or topic?