The paradigm in Milky Way astronomy as of 2018 is that:
The Gaia Sausage or Gaia Enceladus is the remains of a dwarf galaxy (the Sausage Galaxy, or Gaia-Enceladus-Sausage, or Gaia-Sausage-Enceladus) that merged with the Milky Way about 8–11 billion years ago. At least eight globular clusters were added to the Milky Way along with 50 billion solar masses of stars, gas and dark matter.
A preprint announced today confirms that hypothesis:
Our results from kinematical and chemical analysis provide important clues for the assembly history of the Milky Way, especially for the Gaia-Sausage ancient merger. The kinematical and chemical trends found in this work are consistent with that of recent simulations which indicated that the Gaia-Sausage merger is the dual origin of the Galactic thick disc and halo.
I. Ablimit, G. Zhao, U. Teklimakan, J.-R. Shi, K. Abdusalam, "The Milky Way Revealed by Variable Stars I: Sample Selection of RR Lyrae stars, and Evidence for the Merger History" arXiv:2111.00028 (October 29, 2021).
Meanwhile, the data does not conclusively favor a scenario in which Venus was never habitable over one where it once was, or visa versa.