You do not need to finish the main campaign to play the DLC. It can be selected directly from the main menu under the campaign option. 3. Performance & Visuals: Is it Worth It?
The most recent major support cycle for the Switch port culminated in the series. Doom Eternal Switch NSP UPDATE DLC
When installing the Doom Eternal NSP, DLC, and Update files, firmware version is critical. You do not need to finish the main campaign to play the DLC