Hi, Alice. Thank you for responding.
A different approach resolved my issue: rather than detecting the file by date in my trigger, I added an Interval trigger, set to e.g. Daily, and then used an SFTP - List Files action after to act on all files in the SFTP folder. After the List action, we use a SharePoint - Create File action, and finally an SFTP - Delete File action.
This solution was provided to me by Microsoft Flow Support. Very elegant. Now, the only remaining question is, when I use this method to copy 6000 55 KB PDFs from SFTP to SPO, the rate is only 8 files per minutes, under Flow Plan 2 trial -- is that rate the best we can expect?
Nick