I already noticed that the left part of the monitor is much more "yellow" then the rest. Perhaps because it's a used one for a discounted price. I read somewhere that I should check for "A00" through A03", as that indicated when the monitor was build and it's quality. Do you know what exactly that means?
By using HDMI instead of DisplayPort, should I pay attention to specs of that USB-C to HDMI cable or is any with decent reviews from amazon good to go?
As for the sleep mode of the Dell, it does go into it, but the white light is still on, at the bottom right. That seems like standard stand-by mode, right? In that case I should turn it off manually, just to save the energy the monitor would waste if I am away for a few days for example. Doesn't the LG 5k completely shut down if the iMac/MCB goes into sleep mode?
I am receiving Spyder 5 tomorrow and will calibrate both, the iMac and the Dell and let you guys know how I like it. So far, the Dell is fine, but obviously I much rather prefer to work on the iMac and therefore really need to think twice about keeping it.
The factory condition should already got a calibration for uniformity, and by default the "uniformity compensation" feature should already be on, unless you switch to CAL or ICC profile modes I believe. You should go into the Dell's on-screen-display menu to locate this option and confirm. This in theory should root out partially tinted and/or darkened areas of the panel by a large amount. If you are still seeing locally yellowed areas no matter what setting, then it is a lemon (probably why it is discounted I guess). It all depends on how you use the display and if you accept the amount of tint, for instance if you color correct photos/videos which frequent have grey / white scenes then it is definitely not acceptable, but if it is just a media consumption display then you may let it pass.
I haven't followed which specific batch number of the "make" is better than the rest, since I don't own one (I only considered). AFAIK, the earliest June-August batches had obvious issues, where the Nov batch had some issues resolved or have better QA. But these are just what I gathered from all over other display forums on the internet.
I think for 4k 60Hz, the USB-C has to specifically states it supports HDMI 2.0 version, if it is ver. 1.4 or lower then the output is limited to 30Hz on the U2718Q. I myself didn't buy any such dedicated display dongles, as my OWC TB3 dock has mDP port, and then a G-RAID has HDMI 2.0 on it, so I am covered even for a tri-monitor setup for my iMac 2017. But I think 4k through to HDMI is now such a frequently requested feature, and the USB-C situation is better now than say a year ago, most of the newer dongles should serve this well.
Stand-by mode is exactly what (I thought) you looked for, backlight panel + LCD screen is off, but the mainboard of the display stays on always waiting for the computer to wake it up. I don't think the energy savings by totally powering it off is significant enough to warrant the hassle, but if you are offsite for a long time then powering off is probably for the best in terms of household safety. I don't think any monitors out there would shut itself completely whilst in standby mode nowadays, otherwise it would take an unacceptably long time to wake/re-sync from the source, by today's standard.
The Spyder will help if color is a concern in your workflow. But you may already know it, then the U2718Q doesn't cover as much color gamut as the iMac's P3. So if you got a wide color image such as a DSLR RAW, using software that supports wide gamut ICC like Lightroom, then you definitely are going to see more colors on the iMac screen than the Dell. I believe even the Dell HDR or whatever they call it won't beat the iMac screen either, since it has lesser peak cd/m2 on paper.
