it sort of doesn't matter how you author the effect. You can convert bone animation to blendshapes, you can convert any sort of vertex animation (sims, blendshapes etc) to bones - you can convert any of that to a vertex animation stored in a texture that unreal can eat quite happily worry about how to animate the effect…
if you are on the go fast mindest , rig + join + blendshape sounds like a perfect idea. but if you really want to push your studies. i will investigate houdini with - kinefx join chains - vellum post sim with your kinefx anim as target ref geometry - then vertex animation texture for export This is very impressive work…
For something simple it could be an alpha animation in a shader. You model have a bud and full open petals at the same time and the shader makes open petals invisible first, then gradually does it all visible. Wouldn't be same cool looking although .