What are people using to upgrade from Channel D Pure Music to play from iTunes

if there is a need to Still rip CDs I would look at one of the INNUOS ZEN series.
Otherwise, with that budget, I highly recommend the INNUOS Pulse.

As far as purchase look on the INNUOS website for dealer location information. There are a couple of online dealers like Audio advisor, and The music room. 

Buy used.  Here’s an Innuos Zenith you can get for less than the price of a new Zen, but others are available too you can find on hifishark.

https://www.usaudiomart.com/details/650182374-innuos-zenith-mk3-2tb-storage/

Your Mac is fine. Don’t replace good machine with some low power ARM streamer. My Windows box streams just fine, Aurender is not any better so I end up not using it. And beside, why would I want to control some unit from phone when I have 34" screen with Tidal?

Tidal with max quality is better than CD. However, they only stream new, remastered editions. So you may be disappointed if you prefer older mastering. Ex Tony Iommy screwed up old Sabbath mixes pretty good.

I have found Tidal to be somewhat dissapeointng, I switched over to Qobuz.