The anti-collapse move your obliques have been missing.
Drive your hips up and keep them there. Don't let your bottom hip dip toward the floor.
The side plank makes your core resist lateral collapse, which is the one job front planks and crunches never train. The obliques on the bottom side hold your entire body in a straight line off one forearm, working isometrically for every second of the hold. One side at a time means no hiding a weak side, and most people find one exists.
Quality first, 20 to 30 seconds per side with hips high, building to 60. The set is over the moment your hips start to sink, however long the timer says.
Drop to a bent-knee side plank, balancing on your knees instead of stacked feet. Same oblique hold, shorter lever. Progress to full side planks once you can hold the knee version comfortably for a minute.
Directly under your shoulder, so your bones stack and support the position. An elbow drifting forward or back makes the shoulder work overtime and usually ends the hold before your core is even challenged.
