  Quantum spin liquids (QSL) are theoretical states of matter with long-range
entanglement and exotic quasiparticles. However, they generally elude
quantitative theory, rendering their underlying phases mysterious and hampering
efforts to identify experimental QSL states. Here we study triangular lattice
resonating valence bond QSL candidate materials KYbSe$_2$ and NaYbSe$_2$. We
measure the magnon modes in their 1/3 plateau phase, where quantitative theory
is tractable, using inelastic neutron scattering and fit them using nonlinear
spin wave theory. We also fit the KYbSe$_2$ heat capacity using high
temperature series expansion. Both KYbSe$_2$ fits yield the same magnetic
Hamiltonian to within uncertainty, confirming previous estimates and showing
the Heisenberg $J_2/J_1$ to be an accurate model for these materials. Most
importantly, comparing KYbSe$_2$ and NaYbSe$_2$ shows that smaller $A$-site
Na$^+$ ion has a larger $J_2/J_1$ ratio. However, hydrostatic pressure applied
to KYbSe$_2$ increases the ordering temperature (a result consistent with
density functional theory calculations), indicating that pressure decreases
$J_2/J_1$. These results show how periodic table and hydrostatic pressure can
tune the $A$YbSe$_2$ materials in a controlled way.
