We report on measurements of time-dependent \CP violation in \B decays using a dataset of $365 \text{fb}^{-1}$ collected in $e^+e^-$ collisions at the $\Upsilon(4\text{S})$ resonance by the Belle II experiment in the channels $B^0 \to \rho^+ \rho^-$ and $B^0 \to J/\psi \pi^0$. In the decay channel $B^0 \to \rho^+ \rho^-$ we extract a branching fraction of $\mathcal{B} = (2.89^{+0.23 +0.29}_{-0.22-0.27}) \times 10^{-5}$ with a longitudinal polarization fraction of $f_L = 0.921^{+0.024 +0.017}_{-0.025 -0.015}$. We also determine the CP violation parameters $C_{CP} = -0.02 \pm 0.12^{+0.06}_{-0.05}$ and $S_{CP} = -0.26 \pm 0.19 \pm 0.08$. By combining this result with measurements of isospin-related $B \to \pi\pi$ and $B \to \rho\rho$ decays, the CKM angle $\phi_2$ can be extracted. We extract $\phi_2 = (92.6 ^{+4.5}_{-4.7})^{\circ}$. The precision on the angle $\phi_1$ is limited by a possible shifts of $\phi_1$ due to additional loop diagrams. Control channels to determine this shift are $B^0 \to J/\psi \pi^0$ and $B^0 \to J/\psi \omega$. For $B^0 \to J/\psi \pi^0$ we measure a branching fraction of $\mathcal{B} = (2.00 \pm 0.12 \pm 0.09)\times 10^{-5}$ and the CP violation parameters $C_{CP} = 0.13 \pm 0.12 \pm 0.03$ and $S_{CP} = -0.88 \pm 0.17 \pm 0.03$. In addition, we report on the first observation of the decay $B^0 \to J/\psi \omega$ with the branching fraction measurement of $\mathcal{B} = (2.16 \pm 0.30 \pm 0.14) \times 10^{-5}$.