Uplink coverage in cellular networks is constrained by the maximum UE transmit power, making peak-to-average power ratio (PAPR) reduction essential. While DFT-s-OFDM with frequency-domain spectral shaping (FDSS) achieves significantly lower PAPR than OFDM, especially with pi/2-BPSK, the PAPR remains too high for higher-rate transmission. Spectrum extension (SE) combined with FDSS (FDSS-SE) can further reduce the PAPR for higher-order QAM. This paper considers FDSS-SE with parametrized FDSS windows spanning a range of possible power ripples, as well as arbitrary circular shifts of the subcarrier coefficients. We optimize both the frequency shift and the SE size, and show that there exists an optimal SE size for reducing the PAPR and another one for increasing the rate. Analysis and simulations reveal that both optima largely depend on the window attenuation but are nearly invariant in proportion to the bandwidth. While the PAPR-optimal SE size is nearly invariant to the constellation order of regular QAM, the rate-optimal SE size depends also on the SNR. These insights provide practical guidelines for beyond-5G uplink coverage enhancement, highlighting that SE size should be individually configured according to the user's FDSS window and link quality.