The Pi kit you referenced would need either a HAT or a USB DAC. If you decided to purchase a HAT you will need a new case since as @Derek_Wyman mentioned the HAT plugs into the top of the Pi requiring a taller case with openings for all the HAT’s connections.
Pretty sure you mentioned you had a Pro-Ject Pre Box S2 Digital in another thread. It supports USB so you could connect it directly to the Pi without a HAT. When connected via USB Roon will see the S2 and configure itself for it automatically. If you used a HAT with a Coax or Toslink ( optical ) connection to the S2 Roon would see the HAT but not the S2 requiring you to configure things manually.
You also might not be able to take advantage of all the capabilities of the S2 using a HAT instead of a USB connection. On the other hand you may find that you feel the audio quality is affected by the USB connection. I have a Pi with a USB DAC connected to it and I don’t have any issues but your experience may be different.
If you do have issues with a USB connection to your S2 then you will find yourself in the enviable position of choosing between a better power supply for the Pi ( and possibly for the S2 ), a better USB cable, a USB cleaner or a better endpoint. A rabbit hole that operates like a black hole in space, once you get sucked in there is no escaping
I was considering purchasing an S2 several months ago and purchased a Schiit Wyrd USB Decrapifier to use with it since I planned to connect it to a Pi using USB and I had read that some people were having issues with the Pi’s USB connections. Life happened and I did not buy the S2. I procrastinated and let the return window for the Schiit expire and finally decided to insert it between my Pi and Dragonfly Red DAC. Not sure I can hear a difference but at least I’m using the Schiit instead of looking at it sitting on the shelf