How much does the use of an external DAC matter to you? If it’s just for casual listening while working, I’d suggest just taking the audio out from your iMac that’s already your Roon core, and hook that up to the aux in on the Bose system. (I’m presuming the Bose is an active system, and doesn’t require a pre-amp - which I think is correct?)
If you’d like to not use the onboard sound, something like a Schiit or one of the many ChiFi DAC’s (e.g. Topping) hooked up to the iMac USB should do the trick. I have a Schiit Fulla 2 as my desk setup and love it - and it would also give you the added bonus of a good headphone amp at the desk too.