I've not read all the reply's, but i went through this and still going through this.
-I started with a PC based custom computer for dedicated streaming. I thought I'd try a Roon nucleus and it was not an upgrade if anything it was a down grade in sound, i sold it and lost $600 in one month.
-I thought i'd buy a much better streamer so i bought an Aurender A 10 ($5000 later) and still its not really any better than the PC I was using prior if anything its more difficult to use, nice DAC in it i suppose. I find its good but its a pain to use in comparison to the PC.
I've tried the Blue sound node prior to the Aurender and it wasn't better then the Roon Nucleus.
Sound wise I'd try a good DAC first. in my experience the streamer in some cases sound worse then a computer based system, well lets face it they are all computer based systems.
I'd try optimizing your computer first for streaming before a new streamer. Then make the decision about upgrading the computer.
The streamers can give you easier interface though depending on how you got your mac mini set up i.e. tablet control etc.