imho 95% of calculations with calc are accounting and similar, less than 5% of users will use pi() or similar anywhere,
Ah that “imho” again. But if your estimation is correct, then LibreOffice is perfectly fine for most of its userbase as is, otherwise they wouldn’t use it there. Your changes would benefit no significant percentage of userbase, and will hurt most (even if you invent something working).
rounding results makes sense with ‘round’ operands
There’s no way to know which operand is “round”. The very idea is flawed. The problem you try to workaround is exactly that the numbers have some data in any bit of its mantissa, and there’s no property that could tell “round” numbers from “special”. Even trying to count zeroes before the tail in decimal representation would fail, and break deliberate operation with small deltas.