You can only stream hires to Heos using their own app or using its digital inputs from a hires source any other app will use Airplay and that’s restricted to 44.1/16. Nothing can change that, Roon does not support native Heos streaming it’s a locked protocol.
A Nuc won’t help you here unless your Heos soundbar supports USB audio, which I am certain it doesn’t. You just need a seperate Roon endpoint connected to its optical input. You could use a raspberry pi SBC with an inexpensive Spdif output card to achieve this with little expenditure.
When I am using a nuc in the future I will connect it to my DAC first. Than it should be bitperfect without any downsampleing etc.
But first when I save the money I buy a cambridge dacmagic200M first. And after that a nuc. Now I have to do with nvidiashield with fiiok5pro(dac). Shield is converting every muziek source to 24bit/192. Not ideal when you want to play dsd sources.
I am doing a lot of researching this days think the cambridge with a nuc is best sollution. Or there are better once but than you need a lot of money