My mistake. When you said optional transfer switch, I assumed it was a commercial install, and there was also a required transfer switch.
Personally, I think using a DT relay has too much chance of backfeeding power back to the utility during if the relay failed to operate properly.
My local power company wouldn't allow it. See page 12 of this document for an example:'
https://ameren.com/-/media/corporate-site/Files/Reliability/SafeConnectDocumentC0Form.pdf
I would suggest sticking with a listed manual transfer switch. They aren't that expensive.