Aadm,
My printing is run from GIMP, always, and Turboprint installs a plug-in for it so it shows up on the menu. I export into TIFF from my raw editor and then open the image into GIMP. I always work in ProPhotoRGB. Also, I’ve printed on UltraProSatin and it’s great.
Your P100 supports 8.5x25" panorama prints, and RR has the paper, and Turboprint allows you to create a new paper size. It’s all good.
Finally, glad to read you have calibrated your monitor(s). When I first started printed, I resisted calibration and thought I could fiddle my screen to match up. I was always disappointed, until accepting that a photo printer requires a calibrated monitor.