To clarify a bit in more precise words, Ten'imuhou is the Radiance of Fondness, it's just its Japanese name. The Radiance of Fondness, together with the Radiance of Heartrend and Radiance of Strong Heart, fall under the Light of Pride, somewhat similar to how Hyakuren, Saiki and Ten'imuhou all fall under muga. Oni's Radiance from his match against Kintarou was likely misidentified as Ten'imuhou by the Japanese, but had probably always been the Radiance of Strong Heart all along.
The descriptions for the Radiances as given in the extra page in chapter 283 tell about what kind of players can have these Radiances, roughly translated:
Those who know the pleasure of enjoying tennis from the bottom of their heart can achieve the Radiance of Fondness. Those who understand the origin of their strength and know of its fleetingness can achieve the Radiance of Heartrend. Those who become strong for the sake of someone else can achieve the Radiance of Strong Heart.
To go a bit into further detail, I think each of these Radiances tells about where the player draws their strength from. The Radiance of Fondness is simply Ten' imuhou, enjoying something for its own sake. Echizen and Kintarou are the most representative examples of this.
The Radiance of Strong Heart draws strength from fighting for someone, whether that's one person in particular, your team, or the children at the orphanage you used to take care of. This one is achieved by Oni, who has strong bonds with people.
We haven't seen the Radiance of Heartrend yet (though I suspect we will right in the upcoming match), and it's by far the most vaguely described. I personally think it can be best understood by looking at Tezuka and Yukimura.
Both of them have experienced hardship, causing them to be unable to play tennis. Tezuka through his injury and Yukimura through his disease. In essence, they know that the ability to play tennis isn't something that speaks for itself, that it is fleeting, so to say. In his match against Yamato, Tezuka let go of fighting for, sacrificing himself for a team, and decided to fight for himself. In the process, he unlocked what we know so far to be Ten'imuhou, but could as well be a different Radiance after all.
Yukimura may be a more straightforward example. In his doubles match in the pre-WC against Germany, he was caught by the Yips. In order to break out of them, he had to find the reason why he plays tennis-- the origin of his strength, so to say. The reason he found was that his joy of returning to tennis was stronger than anyone else's. However, he hasn't been shown to unlock a Radiance at all thus far.
The general inability to distinguish Ten'imuhou from the other Radiances on a surface level leads me to believe the physical effects are similar, and that it's mostly the psychological origin that makes the difference.
So, with that reasoning, I think Kaoz' theory of deeply experiencing the other Radiances as origins of strength holds ground.
I looked a bit at the match for hints, and found this line by QP:
"The Light of Pride... By breaking it, I will become closer to the Quality of Perfection."
By understanding those motivations and assimilating or overcoming them, I think QP is able to reach the Ultimate Quality.
The coach's line of "Rather, by [having Oni] attacking him aggressively, QP will get closer to the Ultimate Quality." also seems to imply it's about experiencing the Radiances deeply.
