Be wary. Do you ever use it portably? Even if you use it once or twice a month as a laptop, you'll miss it. There's a difference between choosing to dock a laptop to a more comfortable desk setup and having to be tethered.
Same with Apple's ecosystem, it's only when it's gone and you miss your content for example going from your iPhone to your Photos library on the Mac smoothly you'll wish you still had the Mac. Some people are more bothered by this than others, so think about how you'd feel about it, but the ecosystem is doing so much constantly behind the scenes that you might not appreciate it until it's gone. Nothing else in terms of seamlessness exists, unfortunately. I wish it did.
I mention because these two things brought me back to the Mac a few times at great expense and hassle. Windows 10 looks just as good on the surface, but in my experience it was just an illusion. You have a pretty good spec nearly new MBP. Don't give it up is what I say.
But if you must go, I say for a month (no less), run bootcamp with W10 on your MBP, docked. Don't allow yourself to use it as a laptop whatsoever. When the sun is shining and you'd like to sit out the back and work on something, nope, head to your room and sit there (with a desktop it'll be the only choice). Install Steam and your games on the MBP, play them as much as you envisage you will with a gaming PC. Even if you need low settings for the MBP as Macs aren't ideal for gaming. Ban yourself from using macOS at all. Move all your content to the PC partition (or copy it) If you're finding it a great experience, then a gaming PC will improve it. Then it's worth your money.
Then you'll know you truly won't miss macOS or having a laptop.
They're two completely different worlds. Windows has its benefits, gaming is one of the few. Desktops have their benefits, but convenience isn't one of them. Two opposite ends of the spectrum. So please trial it similar to how I've suggested before parting with that phenomenal machine, as you might not like the far off fields which seem green.
Keep us updated. And if you're in the UK send me the eBay link for your MBP, would consider it!