It sounds like you're getting mixed up between two different uses of "pelvic tilt". In the context of the article you linked, "pelvic tilt" refers to rotation (movement) of the pelvis in space, while the femurs are fixed in position. This is what is supposed occur in the movement of a Romanian deadlift - the legs remain still while the ...
I higly suggest to add those exercise to you abs routine:
dragon flag (if you can't do it with straight legs just close the lever by tucking in)
Those exercise really changed my abs game.
And remember that abs are made in the kitchen.
The planche does not require any contraction of the abdominal muscles, so that's extremely unlikely to be what's limiting you. The natural tendency of gravity in the planche is to pull your hips down, causing your spine to flex. This is the same movement that is performed by the abs, so it is actually the opposite muscles that must be activated in a planche ...
You're asking two different questions with two different answers.
What exercises will increase ab strength without much injury risk to the spine?
What exercises will reduce belly fat without much injury risk to the spine?
To get the easy one out of the way, there is no such thing as spot fat reduction (outside of surgery). Abs are made in the kitchen, not ...
Doing a ton of reps certainly won't help in my opinion. At high rep ranges the muscle adapts to endurance. Type 1 fibers improve. The muscle gets smaller but more efficient at doing all that volume.
Whats the best way to increase muscle size? Heavier weight.
I do weighted sit ups. Should be the same 8 to 12 rep range to failure.
If you can do it more than 15 ...