Phew, finally done with this... But gosh darn it, the result is great.

The building process was a huge pain that took several days to complete, due to the constant trial and error having to do with fitting the radiator.
Beforehand, I thought it would be a couple of small cuts to remove a beam at the top/front of the case, to allow for the long radiator (normally only 280 and 360 rads are supported). But it turned out that my wider-than-normal 420 radiator wouldn't fit without extensive metal removal, so extra time was needed for cutting the frame and drilling/readjusting extra holes in the radiator mounting bracket.
If a standard-width 420 rad was used, the casemod would have been a lot smaller in scope, and I'd recommend others drilling very large holes in the mounting bracket, to avoid having to readjust "reasonably" sized holes, as I had to do.
Further, I would recommend using an angle grinder afterwards to remove the inner area of the bracket that blocks the airflow of the fans, as the bracket is optimised for 120mm fans, meaning a lot of the airway is blocked by unperforated metal when using 140mm fans. It's something I plan on doing later.

Disregarding my modding, the case is a bit of a hassle to work with, but the end result is great.
The case looks fantastic (though it's big for an ATX build) and thermals are great too, despite the restrictive radiator bracket.
It's a shame they didn't go for 420mm support, as it looks like very little has to be changed in manufacturing. Hopefully they can improve water cooling support for the next version.
I will OC the CPU and grind the bracket later, but at stock it's very quiet even while running Prime95 and Furmark simultaneously. With a few upcoming changes, it should be nearly silent while gaming.
The 200mm case fans are OK, but no more. They are not capable of silent operation at low rpm, so they'll have to be turned off entirely for this.
The fan control of the Gigabyte board is the worst I've used and I really hope they improve it, as basic features are very buggy, and lots of things don't even do what they say. Fan curves will have to be set up in windows.

Overall I'm very happy with the build, though there are a few details I'll complete later before it's perfect (rad bracket, more SSDs, new casefans).

Further water cooling details:
EK Tube ZM​T Matte Bl​ack 15,9/9​,5mm
EK CryoFue​l Clear
EK ACF F​i​ttings - ​​Nickel (x2​), Black (​4x)
EK AF An​g​led 45° G​​1/4 - N​ic​kel (x2)
EK AF Angl​ed 90° G1/​4 - Black ​(x4)
EK Cable S​plitter 4-​Fan PWM Ex​tended

Case moddi​ng required for radiat​or compati​bility: 10​6cm metal ​cut and fi​led, 8 hol​es drilled​ and readj​usted to mount rad and 3 fans.
Two 500GB 860 Evo drives will be attached on the PSU cover once new cables arrive, as >70cm cables are required for this.