Should I get a Roon Nucleus?

I would also look at a Small Green Computer solution (ie i5 or i9).

Probably a waste of money if your digital source is a bluesound node. Unless you're having delays or some kind of performance problems with the iMac delivering the music to your node, there's probably no reason to get a nucleus. Is there a reason why you thought you might want one?

It will most likely accomplish nothing for your audio as long as you don’t use that computer as a streamer. If you use that iMac as a computer for other tasks concurrently with running Roon and it doesn’t have enough brains to multitask, you will slow down Roon and Roon will slow down whatever else it is you’re trying to run. Potentially. If you run into it, then make the decision. Other than that you should be fine as is.

Oh…and take whatever it is you were going to spend on dedicated roon core, sell your bluesound node and get a good streamer. 

Nothing to gain from what I see.