The 6-minute wall is per-execution, not per-task
Apps Script enforces a hard 6-minute execution time limit per run, regardless of whether you're on a free account or a Workspace account (which bumps the limit to 30 minutes, but the same cliff exists). The error doesn't mean your logic is wrong; it means one continuous call to your function took too long. The fix is to stop thinking of your job as a single execution and start thinking of it as a pipeline of short runs.
The first time I hit this, I wasted an afternoon trying to speed up the loop. Marginal gains didn't move the needle because the data volume was the real problem — 4,000 rows at one Sheets API call per row will always breach 6 minutes. The correct frame is: how do I save where I stopped and pick up there next run?