1024 is the highest buffer iOS devices can use. Higher than that has only effect on macOS.
If you already get audible artefacts during testing, there is only one way forward: reduce the number of stems. Mixdown stems where you don't need separation to multiple outputs or live balance control.
Avoid speed & pitch changes in Stage Traxx. Bake them into the files in your DAW.
The only other alternative would be a faster device. The iPad Air 3 has an A12 CPU which is quite slow by todays standards. Even the current entry level A16 iPad is more than twice as fast.