Systematic calculations of $\alpha$-decay and spontaneous-fission (SF) properties are presented for the even-even superheavy isotopic chains with $Z=120–130$. The $\alpha$-decay half-lives are evaluated within the density-dependent cluster model using a double-folding potential with a zero-range exchange approximation (DFM-ZR), employing the WS4 $Q_{\alpha }$ values and deformation parameters $({\beta }_2,{\beta }_4)$ as inputs. The microscopic results are compared with the BKAG, UDL, NRDX, Horoi, and Ren A empirical formulas for both spherical and deformed daughter-nucleus configurations. To quantify the agreement between the microscopic and empirical predictions, the root-mean-square deviation, mean absolute deviation, and maximum absolute deviation of ${log}_{10}{T}_{1/2}$ are evaluated for each isotopic chain. The calculations show that nuclear deformation has a significant influence on the $\alpha$-decay systematics, particularly in the heavier chains where the daughter nuclei develop stronger prolate shapes. The DFM-ZR predictions exhibit trends similar to those of the BKAG, NRDX, Horoi, and Ren A formulas, whereas the UDL formula yields systematically shorter half-lives and larger deviations from the microscopic results. The predicted systematics further indicate a structural evolution from weakly deformed nuclei near the vicinity of the $N=184$ shell closure to more pronounced prolate deformations at higher $Z$. Local irregularities around $N_d\approx 194$ in the $Q_{\alpha }, S_{\alpha }$, and half-life trends suggest a possible deformed subshell effect, although a more detailed shell-structure analysis would be required to confirm this interpretation. Comparison with the SF half-lives shows that spontaneous fission becomes increasingly competitive with increasing proton number and is predicted to dominate for many nuclei with $Z\ge 126$, whereas the $Z=120–124$ chains remain more favorable candidates for observable $\alpha$ decay. These results provide a consistent theoretical framework and useful guidance for future searches for superheavy nuclei beyond the presently known limit of the nuclear chart.

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• Alpha decay
• Nucleon distribution
• Spontaneous emission
• A ≥ 220
• Cluster models