Hi Allen, welcome to the forum.
Do I understand you correctly that you are thinking about replacing your digital chain with a Roon compatible setup, but want to keep the pre, the amp and the loudspeakers in place?
And that you want to achieve better sound quality (SQ) with this? You stated that the sound today “is not superb”. This makes me wonder if you identified the cause of the lack of SQ? Can you describe what you are missing? What qualities you want to achieve? Is it really your digital source? Does e.g. CD or a turntable connected to the NAD sound very much different? If yes, then let’s dive into Roon etc. But if not you might want to optimise the amp/loudspeaker system first, e.g. by a different room setup, speaker placement etc.
Regarding your questions:
The Roon core can perform the first unfold of MQA. The second unfolding is always done in the DAC. If you want to do full MQA, the DAC needs to support it. The Topping seems to not support MQA-in with I2S, only via USB. (Alternative to Tidal/MQA is using Qobuz, where native high-res is supported.)
The Roon core does not do Dirac. If you want to optimise SQ with Roon DSP, you can e.g. use the (parametric) EQ, or a convolution filter generated e.g. by REW. But all these options require more work than the probably easy Dirac-setup of the NAD.
A Roon endpoint / bridge can be an RP4 and many are very happy with this solution. Others claim that more sophisticated endpoints (e.g. SOtM, Lumin, Auralic, dCS) have a positive effect on SQ, but only your ears will tell you, what’s true (for you).
Roon core has identical functionality on Windows, MAC or ROCK. Choose by your budget and how comfortable you feel with the OS. Roon Nucleus is a turnkey solution, but when you just start with Roon, why not just use any computer available.
If the DAC is “Roon ready” you can be sure everything always just works as intended, and it may offers some features like digital volume control. But any DAC can be connected to a Roon endpoint, you then just have to manually configure some aspects like bitrate etc. The Topping btw. has balanced out, but the NAD has no balanced inputs, so you would need to connect via RCA. Or do you plan to use the Topping as the preamp and replace the NAD?
It’s always more questions in the end
Hope that helps a bit.