Yeah he was indeed sick of drawing by the end of Kaguya which is why he takes solely the author role for Oshi no Ko while someone else is drawing those manga chapters, so while you could have used this as a main explaination for the drop of quality of Kaguya Samas final arcs, this can't explain Oshi no Ko's drop in story quality.
Frankly speaking, Aka's drop in quality is caused by his lack of discipline. Kinda like Lolda in Wano, he promoted unserious things instead of giving the fans what they wanted and needed.
After the rooftop confession and christmas arc in Kaguya, fans wanted more progress between Ice Kaguya and Shirogane, Ishigami and Miko, and also possibly Chika and Hayasaka in the second half of the story. What we got? Shirogane in Kaguya family drama, Ishigami simping for ages and ages and ages for his Senpai with Ishimiko being completely offscreened, and several panels of Chika and Hayasaka (also off-screened).
He did this again with the second half of OnK (basically everything post-Tokyo Blade arc). His only good work is everything involving Kana, meanwhile Ruby became unlikable, Akane had to be a satellite because she is too smart, and Aqua just meandered from point to point with no obvious progress in his revenge. The moment we got to the Wano of OnK, the film arc that has been built since the first chapters, he flunked it HARD just like Lolda flunked Wano. We got no significant substance of what the film is even about (beyond what we already knew) and now the arc is forever known instead as the incest arc.
Basically, Aka went full tin-foil in both instances. He simply couldn't write a decent second half, I think.
And this is also why, although I am upset by the cancellation of Love Agency, I was not really surprised and I'm not really sympathetic toward the fate of his new work. Ain't no way he could write them at the same time while fumbling from arc to arc like that. It's starting to make me believe that Aka bought into his own hype and thought he could do no wrong after his work found early success, that's why his second halves sucked.
