Don't see them using separate two-letter booking classes any more for the different levels. Receipts all show only O or N booking classes for DL awards. The underlying fare basis codes of the awards (from which the mileage levels are derived) are listed in the Fare Details: section of the email receipt. Awards on partners can book into different booking classes.
DL metal flights still use two letter fare classes. If you book in the app it shows you the two letter code during booking. The receipt always just shows O/N.
Partner awards book into whatever the partner uses. For Skyteam airlines this is O for business, and N/X for coach. Other partners uses different fare classes.
As for OP, if the DL metal flight isn't available at the lowest level then pricing becomes additive. It means you have to book an award for TPE-JFK and a separate award JFK-DCA. If JFK-DCA is available at the lowest "saver" level then you can book one award TPE-DCA instead.