I'm planning to get the i5-8400 with Intel UHD Graphics 630, and I'm not sure if buying a dedicated GPU will actually work. (By the way, I'm not changing the CPU, so don't say screw the i5-8400 and get a different one that doesn't have integrated graphics.) Anyway, is there a way I could somehow 'turn off' the integrated graphics? Or just bypass it maybe? Any help is fine.
You can combine a CPU with integrated graphics with a discrete GPU. There's an option in BIOS to disable integrated graphics of the CPU but many Motherboards do this automatically so you don't have to worry about it.
Every motherboard will allow you to use a discrete graphics card even if you have integrated graphics. Most should do it automatically, some might require you do do it manually but it doesn't really matter because it's trivial to do.