I must respectfully disagree there but obviously it might not work out for everyone.
I can hear a decent difference with my Lumin T2 and Nuc with Rock compared to previous Dell win10 laptop and ultrarendu combo.
Yes it cost a decent amount of money more as well but I have zero issues ever and truly enjoy listening to my music through it.
However back to the ops original question.
I believe it would be worth investing in a SSD to at least bring the Mac up to Roon minimum requirements and give that a whirl before even thinking of anything more esoteric.