For anyone struggling with this, the issue was very simple and dumb. Gigabyte has changed the format of some bios entries from an "Enter" with a context menu popup to a page up/down format. Simply highlight the cpu multiplier entry where it usually says "Auto" and use page up/down to select your multiplier (50 for 5.0gHz). It allows up to 52.
No it's not just the 8700k. I have the Z390 Designaire with a 9700k. Easy OC is "Not Supported here" in BIOS (F6) with the 9700k which makes me think it's not the CPU's locking us out, it's GIGABYTE. SO, I thought, okay, maybe they want me to do it through their "App Center" so on Win 10 I downloaded Easy Tune, EVERYTHING is greyed out. The only way I have been able to OC at all is through Intel Extreme Tuning Utility, and that's only on the windows side, so for my Mac OS Mojave install I get no OC.
What the heck is 12+1 phases worth if they won't let us OC at all???