Whomever said a 7700k was too slow and would bottleneck the gpu was full of (insert favorite flushable terminology here, mine starts with S and ends in T).
There's only 2 reasons for such an announcement, 1) whomever said it was totally clueless, 2) whomever said it was hoping to scare you into purchasing an expensive upgrade through them.
The fact they used the term bottleneck is a dead giveaway, full of (and again, the insert) because it's impossible for a cpu to bottleneck a gpu. A bottleneck is a component that slows down the flow of information, choking the output. That's like a kink in a hose slowing down the flow of water. BUT, that assumes the flow is actually higher to start with. The cpu cannot be the kink, it's the tap. It supplies all the fps output, it's not in the middle, it's the start. So it's output is whatever it is, it slows nothing down.
And that changes from game to game. There's a large difference between the fps output capability in CSGO and BF5, and that difference affects any cpu, even the brand new grandaddy i9 10900k or the diminutive i3-6100.
That fps gets sent to the gpu, which either lives upto the limit, or fails, depending on the strength of the gpu and level of details and resolution. The cpu output has no affect on gpu ability it only affects the amount of work the gpu has to do. A cpu will not 'slow down' a gpu at all.
If you have a Dell, that's it, you are done. There's no simple 'I'll just swap the motherboard', that doesn't work. You own a Dell, and Dell owns the proprietary rights to the Windows. It's tied to the motherboard. If you change the motherboard, you no longer own a Dell, you own an aftermarket pc that's housed inside a Dell case, go get your own version of Windows, the Dell version key will not be valid and any warranty, written or implied, any support from Dell at all will be done and over with.
The i7-7700k was the flagship cpu, there isn't one better or stronger in the lga1151 v1 generations. And honestly it's not all that weak, it's plenty capable of very high fps, no matter what game, it's only a few % weaker than 9th gen cpus like the i7-9700k. It'll pair just as easily with a gtx1060 or a RTX2080ti, how much use you get out of the gpu depending on what resolution, 1080p or 1440p or 4k, and the level and complexity of graphics details.