Dendrite formation during electrodeposition while charging lithium metal batteries compromises their safety. Although high-shear-modulus (Gs) solid-ion conductors (SICs) have been prioritized to resolve the pressure-driven instabilities that lead to dendrite propagation and cell shorting, it is unclear whether these or alternatives are needed to guide uniform lithium electrodeposition, which is intrinsically density-driven. Here, we show that SICs can be designed within a universal chemomechanical paradigm to access either pressure-driven dendrite-blocking or density-driven dendrite-suppressing properties, but not both. This dichotomy reflects the competing influence of the SIC's mechanical properties and the partial molar volume of Li+ (VLithorn I) relative to those of the lithium anode (GLi and VLi) on plating outcomes. Within this paradigm, we explore SICs in a previously unrecognized dendrite-suppressing regime that are concomitantly `soft', as is typical of polymer electrolytes, but feature an atypically low VLithorn I that is more reminiscent of `hard' ceramics. Li plating (1 mA cm-2; T = 20 degrees C) mediated by these SICs is uniform, as revealed using synchrotron hard X-ray microtomography. As a result, cell cycle life is extended, even when assembled with thin Li anodes (30 mu m) and either high-voltage NMC-622 cathodes (1.44 mAh cm-2) or high-capacity sulfur cathodes (3.02 mAh cm-2).