I'm using a 750D and I'm super happy with it. I have 6 fans and only 2.5inch drives so I removed all the cages on it and the airflow is really really good and it can get super silent as I use my Grid+ to adjust the fans. I love that little gadget, best 30$ I ever spent
I do use Corsair keyboards, but that's really about as far as I go with them anymore. My biggest problem was when I bought my 900D and found that it not only had manufacturing defects, but also poor quality control. That stung a bit since the case cost me around $350. The 750D doesn't look bad, but I think one thing that I'd like to see in cases is 3x140s in the front rather than 3x120s or 2x140s. I tend to run my fans a bit slower, so having more fans means I still get modest airflow throughout the case.
I have a Define S which supports a lot of fans and cooling options but you might want to consider the Meshify C and Silverstone RL06 if you're looking for lots of airflow.
The Define S might not be a bad option. I removed the 5.25" drive bay from my Define R5, which pretty much turns it into a Define S. The S can also fit 3 140s in the front, which is something that I like.
I think those other two fall prey to one of the fads that I think tends to be more detrimental: PSU shroud. My issue with PSU shrouds is that while they can clean up a build's look, they often fit very close to the bottom expansion bay. This means putting a PCI-Express card at the bottom will have very little space. I had an AIO-cooled 1080 Ti in a case (pumping its air out the back) and I decided to toss an air-cooled 1080 at the bottom. Yeah... it didn't do very well. In my Phanteks case, the PSU shroud causes there to be little room for running cables near the bottom of the motherboard (when the motherboard is installed), which is usually where the USB 3 connectors are.
I've got that same 1080 in a Phanteks Enthoo ITX case right now, and I'm a bit worried about its performance given there's maybe 1/4" between the card and the PSU shroud.
Anything with front and/or side intakes that pass airflow directly over the GPU is critical for high end GPUs. Those two are good on that front.
I'm a pretty big fan of side-panel fans. I own a few Define R5s, and I use the side panel fan on all of them. The only downside is that you should
really use a fan extension cable to make it easier to disconnect the fan if you want to take off the side panel. Also, I highly recommend tossing a magnetic filter on the side, which works pretty well. I look at it the same way as you... it provides cooler air directly onto the GPU.
I have a Corsair Carbide Air 540 and I seem to get plenty of airflow.
The airflow in the motherboard chamber wasn't bad, but I did have issues with the hard drives being in the motherboard chamber. I noticed that the left HDD got a modest amount hotter than the right one. I don't know if it had to do with the video card dumping heat into the case or something, but it did bug me. I looked at the 740, but it seems like it puts the 3.5" drives in the side chamber, which has no fans.