IMO your existing DAC is quite good https://www.stereophile.com/content/nad-c-328-integrated-amplifier-measurements , why buying another DAC...
If you are OK with TOSLINK (typically up to 96kHz only), a reasonable PCI-e soundcard with TOSLINK output should do just fine, like e.g. https://www.pccasegear.com/products/45893/asus-xonar-se-5-1-pcie-gaming-sound-card
If you are OK with TOSLINK (typically up to 96kHz only), a reasonable PCI-e soundcard with TOSLINK output should do just fine, like e.g. https://www.pccasegear.com/products/45893/asus-xonar-se-5-1-pcie-gaming-sound-card