FYI – 9900K still works beautifully, still better than my 10 core. Still better than my colleague’s 12-core. Still better than my other colleague’s 16-core. Still better than any Mac I ever owned, by far. That’s real-world, no synthetic benchmarks. Still the most enjoyable DAW hardware I’ve ever used. And I’m still surprised. The core count vs clock speed vs multithread profile of DAWs in general plus the cost factor, make this the current perfect sweet spot IMO. I think spending more money on a better CPU is counterproductive based on my experience and my unscientific tests so far, for very little gain, if any. And that applies to other DAW apps I use as well. And even for video editing, if you go look up real-world performance ratings, the 9900K is really only beat by spending significantly more money for very little gain. But what matters is your own usage profile, so I can’t speak to your specific needs. For me and my projects, it honestly beats anything else I’ve used. It’s been very surprising to me. Then you can spend the extra money on microphones and other goodies for your studio.
Anyway, to answer your question, motherboard is the Gigabyte Z390 Designare (I thought it had a great balance of features for a DAW and good price, plus Thunderbolt 3 in case I want to try it, which I haven’t), graphics card is currently GTX1080, 64GB RAM, all-SSD (6+TB of SSD total), and RME. No overclocking so far, no need, no desire to overclock. But good to know I can later if needed. I keep the OS clean, simple and optimized just for DAW use, didn’t need much tweaking. One of the easiest builds I’ve done in recent years, been nothing but surprised at how smooth things have been working so far. Who knows, maybe I got lucky, but this is a little golden machine IMO. Running Windows 10 Pro 1809. I was skeptical to go with Z390 chipset since I’ve been on Xeons for a while now, but this runs my DAW software better than any system I have ever owned or of people I know with far more expensive rigs, and it cost 1/2 or 1/3 of my last build. Hope that helps.