Just have live preview treat admonition boxes the same way it treats quote boxes and render latex in them.
Current workaround (optional)
If the math is sufficiently complicated that I don’t want to deal with not being able to see a preview, I sometimes write it first as a quote block and then only add the [!Proof] part when I’m done.
It renders it fully and correctly in the live preview when your cursor is outside it, but while you’re editing the admonition it displays in live preview basically the same way a quote block does, except for the fact that it doesn’t render the latex.
Also, I wrote another plugin that focuses on this specific problem, so I will remove this feature from LaTeX-like Theorem & Equation Referencer in the near future.