Great, looking good!I tried to apply multiple pieces of advice - doing the kata right after a hard workout, keeping the back loose, expansion/contraction - but it doesn't seem to have done much.
If even this kata ends up being stiff or tense, I'm just gonna either a.) just stay tense and cope with it or b.) quit doing kata altogether and thereby abandon competition. There is no conceivable way I can be more relaxed than this.
Password is "kata" again.
Just know it won't be a one quick fix sorta thing. Relaxation is something SO many people struggle with, and it's been one of my primary focuses the last few years. So it may take some time, just keep in mind as an intention that you want your karate to be more natural, fluid, with a sense of ease. This doesn't mean floppy, still with a balance of soft and hard, but think of it like the points of tension more as points of "connection". And that they are merely points of inflection along a continuum. Like one line of music that rises and falls, with differing emphases along the way. Kata are so similar to pieces of music...
Even think of relaxation as your base operating platform. That's your foundation upon which all technique comes from and returns to. It's helpful these little symbolic analogies as it makes certain imprints into your consciousness, and we can often relate to them better than simply practical instructions of "do this, don't do that." And like @isshinryuronin said earlier, it's very much more about "feel". Have those slower session where you explore this deeply.
Seriously, don't give up. This line of inquiry of learning naturalness and relaxation will extend far, far beyond just karate and will legitimately influence the rest of your day-to-day life.