How to summarize the contributions of co-authors where they advised at all stages of the study, but technically only stepped in at the writing stage
All my coauthors have provided advice from conception to completion, on the study itself, data analysis, write-up...If I said: &quot;All authors contributed to conception of the study, data collection, and analysis.&quot; This would be over-selling co-author contribution.

I disagree. That's exactly what your co-authors did. Execution and writing needn't be the responsibility of all authors, especially when one author is a PhD student.
One way to better appreciate the efforts of others is to ask whether work could have been completed without their input. When the answer is yes, you could consider whether their efforts are worthy of acknowledgement rather than co-authorship. When the answer is no, it is surely clear they played an integral role and merit co-authorship.
Returning to the OP's example, you could ask: Could the study have been conceived without all authors? If so, then perhaps attribute conception to only those that were needed. Ask the same for data collection and analysis. You can then write a more accurate contribution statement. Personally, I'd be generous. Err on the side of inclusion.