Yeah, PLA can be quite brittle, especially for thin, load-bearing features like those hooks. Since the hooks failed under flexing, I’d focus on wall thickness first—bumping it to 3 mm around the hooks would add stiffness where it matters most. Infill helps more with compressive strength (like screw holes), but won’t prevent flex cracks as effectively.

If you’re using localized thicker walls (3 mm) at the hooks, you could probably keep the general infill at 20-25%—the walls will handle most of the stress. Just make sure layer adhesion is good (print hot, ~210°C, with minimal cooling). Also, orient the hooks so layer lines run along their length, not across—that’ll help orient the delamination risk in your favor.

1–2 mm fillets at the base will reduce stress concentration. If it still cracks, try annealing the PLA (oven at 100°C for 30 mins)—it can improve layer bonding.