  Spin-boson models are simple examples of quantum dissipative systems, but
also serve as effective models in quantum magnetism and exhibit nontrivial
quantum criticality. Recently, they have been established as a platform to
study the nontrivial renormalization-group (RG) scenario of fixed-point
annihilation, in which two intermediate-coupling RG fixed points collide and
generate an extremely slow RG flow near the collision. For the Bose Kondo
model, a single $S=1/2$ spin where each spin component couples to an
independent bosonic bath with power-law spectrum $\propto \omega^s$ via
dissipation strengths $\alpha_i$, $i\in\{x,y,z\}$, such phenomena occur
sequentially for the U(1)-symmetric model at $\alpha_z=0$ and the
SU(2)-symmetric case at $\alpha_z = \alpha_{xy}$, as the bath exponent $s<1$ is
tuned. Here we use an exact wormhole quantum Monte Carlo method to show how
fixed-point annihilations within symmetry-enhanced parameter manifolds affect
the anisotropy-driven criticality across them. We find a tunable transition
between two long-range-ordered localized phases that can be continuous or
strongly first-order, and even becomes weakly first-order in an extended regime
close to the fixed-point collision. We extract critical exponents at the
continuous transition, but also find scaling behavior at the symmetry-enhanced
first-order transition, for which the inverse correlation-length exponent is
given by the bath exponent $s$. In particular, we provide direct numerical
evidence for pseudocritical scaling on both sides of the fixed-point collision,
which manifests in an extremely slow drift of the correlation-length exponent.
In addition, we also study the crossover behavior away from the SU(2)-symmetric
case and determine the phase boundary of an extended U(1)-symmetric critical
phase for $\alpha_z < \alpha_{xy}$.
