I own a Raspberry Pi model 3 B, and it's quite a fun little box. But a couple of things to note about performance: a slow SD card and the built-in wifi made it slow and glitchy. Moving over to the fastest SD card I could get, and an ethernet connection made it much better. It even worked nicely with my Audioquest Dragonfly. However, the last time I checked out AirPlay software for it, there was some problem with the mixing / signal processing, and I wound up with some digital clipping. Unfortunately, when you're relying on volunteer-based free open-source software, it can be slow and difficult to get bugs fixed. I kept it on Raspbian, not being a fan of OSMC. If you're going that route, make sure to get a package that includes heatsinks.
For what it's worth, I've been considering going to a Bluesound Node 2i. It does what I want, without being fussy, and you can hook it into whatever DAC you like. With the Raspberry Pi route, you do need to ensure the proper drivers exist if you're using a USB DAC. It looks like you can get a SPDIF out board for the GPIO connector on the Pi, but at that point, I'd start considering the time and effort you're putting into it along with the fact that you're half-way to the cost of a more purpose-built streamer.