JF Dave Collins has been for several years now doing a technique that a Luthier at Elderly came up with that when "appropriate for the instrument" meaning it's not a pre-war Martin

is an option and would work well for a wavy crack.
You're right when they don't close with humidification and stay closed in 45% RH filling with something is what we do too.
So forgive me please but it's quality epoxy, Dave will use several, West, SIII, Bob Smith, etc. with color added to match the top. He glues a BJR (back joint reinforcement) strip under the crack to contain the epoxy and then carefully fills the crack with a color matched epoxy fill. The BJR under the fill is doing the heavy lifting and is really a cleat of sorts.
At times it can barely show but it always will show a bit there is no way to get epoxy to look exactly like spruce. Splints can show too and often do.
But it's an option if the instrument would not be devalued by this technique.