I think there is a misunderstanding here as to what the SI units system is all about and how it is used. The purpose of SI is to define a set (of seven) orthogonal units, all defined in terms of fundamental physical properties which could be, in practice, determined anywhere in the universe at any time by anybody, given the definition and the ability to understand what it meant.
This does not mean that only these “SI base units” are to be used in expressing measures, and nobody uses them that way. When working with measurements in various fields, one chooses derived units, defined in terms of the SI base units, which are appropriate for the application. When working with wavelengths of visible light, one typically uses nanometres. When measuring liquids, litres (0.001 metre³) makes sense, or millilitres for small quantities. Astronomers use astronomical units, defined as 149,597,870,700 metres, and particle physicists express energy as electronvolts, 1.602176634×10^{−19} joules. Many units for other physical quantities can be defined as the product of base units, for example the volt is defined as (kg m²)/(sec³ ampere). The point is that the definition of the unit employed is always referenced back to the SI base units and can be easily converted through them as necessary. Because the definitions of the SI units are in terms of fundamental constants such as the speed of light, Planck’s constant, and the charge of the electron, as improved instrumentation and measurement techniques permit determining these values more accurately in the future, all of the units referenced to SI gain accuracy automatically.
As the U.S. customary units have been, since 1959 (and in large part since 1893), defined in terms of SI units or their metric predecessors, with exactly defined conversion factors, they can be considered as simply another alternative set of definitions based upon SI. When the SI kilogram was redefined in 2019 based upon Planck’s constant, the second, and the metre, that improved and more stable definition automatically became the definition of the U.S. pound(-mass).