With Deku’s intelligence, and that he usually doesn’t start full send 100% in fights at first if I’m not wrong, he’d noticed that Geats is able to match him toe-to-toe even as he raises his power and speed. And he’d find it strange that Geats would only go equal, but not above him in strength. With the help of vestiges in analyzing Geats, Deku can conclude that Geats has the ability to match equally his power and speed, not more.
Thus, Deku would save his trump cards, Fa jin and especially gearshift, to only when he has a decisive move to victory, trying to prevent Geats from also matching his powers with the two quirks.
Tho this is only in terms of how Deku usually fights iirc. Actually against Geats’ skill is another matter entirely.