Looks like you are headed in the right direction with a USB-to-SPDIF converter. There are a number of them on the market that work well. Also, if budget permits and you are using software that supports streaming (Roon, Tidal, Amazon) them something like an Auralic Aries would be a great option, and probably upgrade in quality.
To reinforce, though, using standard consumer sound cards or the SPDIF output on the motherboard is not considered a best practice. Some prosumer cards and those that support ASIO drivers are an improvement, but really you want your digital stream not to be affected by all that noise inside the PC - something external is generally considered better.