Hello! Thank you very much for your kind words.
I think for something that's easily poseable like VRoid, you should always seek for 3d-based apps like VRoid itself, or Poser, or maybe Koikatsu (for something anime-styled). Due to the nature of being 2D, there's actually no magic involved in Mannequin; every pose, every clothing, every face parts and hairstyles, are drawn manually. More poses are planned for future versions of course, but because everything is done manually, expect it to take quite a long time.
For more pose flexibility, we have nothing planned in the short term; in the future we might add Godot and/or Unity 2D character export with rigged bone system, but it's still a long way to go.