Let me say something else, why do we assume Shigaraki is Low 7-B in AP? Because the doctor, Endeavor, and Aizawa compared him to All Might.
"That power, it's just like..." Endeavor while flashing to All Might in his head.
"It's not quite on par with All Might, sadly! Excessive remodeling would've overburden his brain." The Doctor to Present Mic. (The Doctor's statement takes priority over all of them, since he's the one who made Shigaraki this strong, and knows All Might's strength better than anyone)
"As powerful and sturdy as All Might." Endeavor to everyone else.
"But as long as I don't blink, that power is all he's got. And I'll hold on for a good long time." Aizawa agreeing with Endeavor.
This all happens before Shigaraki stops "holding back" as you say. These statements are the only reason Shigaraki is being consider to be Low 7-B, without them he'd be 8-A with Low 7-B durability. Endeavor clearly believes he could take a hit from an All Might level opponent, and Aizawa believes they can handle that strength. Which is true since Shigaraki was losing, he couldn't win as long as Aizawa was erasing his Quirks. That's why he started out with going after Aizawa instead of finishing Endeavor off, and then later decided that he has to grab Izuku and run away.
Trying to say that during this fight, at any point, Shigaraki could've used less than 1% of his strength to one shot Endeavor is absurd. Since he has no in character reason to hold back so much against Endeavor. I do agree that he's not hitting Endeavor with 100% of his power. However, I refuse to believe that he's holding back so much that he's purposely keeping Endeavor alive for PIS.
Also isn't funny how we started off this arc with people saying Shigaraki doesn't scale to Endeavor, know we're saying Endeavor doesn't scale to Shigaraki.