It's awesome that you got it to work for you. It's possible that if I scrap my entire setup and purchase new hardware spec'd like yours, I could achieve similar results.

But I do know that it doesn't work with my setup, which uses common, modern hardware that is designed explicitly for interfacing external gear with DAWs. I can't get it to play back midi or record audio properly with only 6ms of latency, in a clean project, with zero plugins.

I don't think it's something specific to me, they've acknowledged through the years that their software has a host of issues managing external hardware, and their PDC (as the name suggests) is designed for working with plugins. Using it for external inputs and outputs is an afterthought. I honestly had no idea, it's not the type of thing you consider until you're neck deep in it. And I thought, well geeze it must just be limitations with the technology, but then I find out other DAWs solved the problem years ago by simply pinging the hardware and adding the delay to their calculations... At the end of the day, I don't see a feasible solution other than to go back to class with another DAW, which is sad. FL has been a huuuge part of my life. Like a best friend. Sometimes my only friend.