Elbow lever. Basic breakdancing freezes.
Pure bent arm work doesn't help.
Which is why I suggested HSPUs...?
That said, bent arm work
does help. The various freezes I've learned in breakdancing got me to use my shoulders, core, and various stabilizers in ways I'd never imagined. When you combine that with things like HSPUs, it sure does help a lot with planche work. I can do the advanced tuck planche now with relative ease, which I could not do 3 weeks ago, and I haven't attempted a single planche in that period.
Of course, you want to focus more on strength than stability for the planche, but that doesn't make the latter useless.