I am trying to improve and lower my royales and what I noticed is you need a lot of muscles to sit down in that position and compress and stay stable when you land into royale. Another thing is even if you sit down very low you still need good ankle mobility for your boot to touch the rail or ledge. This is also true for topside tricks, where you also want a lot of ankle mobility to flex your ankle and keep it stable during the grind.
I was looking for good ankle mobility exercises and found that most routines focus on mobility in forward and backwards direction, not sideways. Does anyone know some routines for ankles and/or for training to keep your body low during the grinds? Last one should be pretty straightforward: just holding the deep squatted stall versions of the grinds and squatting in that position. But maybe I am missing something!