How was he not interested in fighting? Where is the evidence of that.
Now, you know the thing I mentioned earlier about stamina dropping when surprised. Yhwach is surprised when Ichigo's Blut Vene activates, and after being surprised, he takes damage from Ichigo.
Besides this, we know Ichigo can strengthen depending on his emotional state. He's angry because of what happened to Soul Society. The aura he emits when he's about to launch his first attack against Yhwach confirms that he has a rage buff.
So, the Ichigo who can burn Yhwach's arm is much stronger than the Ichigo who fought Quilge. This is because he has both a rage buff and a quincy buff. In the fight with Quilge, he only used his quincy power to break the Jail.
After burning Yhwach's arm, he says, "Did you think you won just because you dodged my attacks?" This is clearly confirmed by Yhwach's surprised face when he thinks he's won and turns away, but at the last moment, Ichigo's Blut Vene activates. Indeed, Yhwach prevails again later in the scene.
We should also remember that Yhwach wants him to join his side. How logical would it be for him to kill or injure someone he wants to win? His words, "I'll drag you away, even if it's the hard way," and even his final words, "I'll come back just for you," prove this.
All things considered, if you say Yhwach and Fullbred Ichigo are in the same league, that would put Pre-Aushwalen Jugram on this level, as he was able to cut through Fullbred Ichigo's sword. But then Jugram loses to Sealed Tenjiro. Base Yhwach, on the other hand, can fight Ichibei, and Ichibei is an unsealed member of Squad Zero. This would clearly cause circular scaling.
No, because Yhwach was asleep and was toying with them that logic doesn't apply to Ayon who is a raging beast that was actively trying to kill them.
What about Starrk's example? His ceros are powerful enough to damage Love and Rojuro. But he stated that his ceros couldn't kill them, so he felt compelled to use wolves.