The effective double-site metal ion replacement strategy was adopted to optimize the crystal field environment of a Mn4+-activated fluoride phosphor. In this study, a series of K2yBa1-ySi1-xGexF6:Mn4+ phosphors with optimized fluorescence intensity, excellent water resistance, and outstanding thermal stability was synthesized. The composition adjustment includes two different types of ion substitution based on the BaSiF6:Mn4+ red phosphor: [Ge4+ → Si4+] and [K+ → Ba2+]. X-ray diffraction and theoretical analysis revealed that Ge4+ and K+ could be successfully introduced into BaSiF6:Mn4+ to form new solid solution K2yBa1-ySi1-xGexF6:Mn4+ phosphors. The emission intensity enhancement and slight wavelength shift were detected in different cation replacement procedures. Furthermore, K0.6Ba0.7Si0.5Ge0.5F6:Mn4+ with superior color stability performance possessed a negative thermal quenching phenomenon. Excellent water resistance was also found, which was more reliable than K2SiF6:Mn4+ commercial phosphor. A warm WLED with low correlated color temperature (CCT = 4000 K) and high color rendering index (Ra = 90.6) was successfully packaged by using K0.6Ba0.7Si0.5Ge0.5F6:Mn4+ as the red light component, and it also exhibited high stability for different currents. These findings demonstrate that the effective double-site metal ion replacement strategy can open up a new avenue for designing new Mn4+-doped fluoride phosphors to improve the optical properties of WLEDs.