Manipulating topological states is a topic of vigorous research. However, the impact of such topological phase transitions on the quasiparticle dynamics remains elusive. In this work, we present the effects of a transition from a strong to weak topological insulator in HfTe5 as a function of Te vacancy concentration. We observed a significant transition from distinct sharp surface states and Dirac crossing to a Fermi-liquid-like quasiparticle state in which these surface-localized features are heavily suppressed. Additionally, by inducing the same effect through applied uniaxial stress, we demonstrate that changes in the lattice constants play the foremost role in determining the electronic structure, self-energy, and topological states of HfTe5. Our results demonstrate the possibility of using both defect chemistry and strain as control parameters for topological phase transitions and associated many-body physics.