[zgs]

I used decNumber in the WP 34S project (https://sourceforge.net/projects/wp34s/) and more recently in the C47 project (https://47calc.com).

You are correct in that the decNumber library doesn't support trig operations. Arithmetic, log, exp and square root plus rounding and some conversion functions.

My experience is that decNumber is generally slow. Logarithms are especially slow. Intel's decimal library is much faster & as you noted, it uses binary operations to start it's algorithms.