I used to write my own .c program to do taxes, the gain being that ongoing revisions didn't require me to repeat any computations ; the downside being that I had to follow the directions. (coding itself is easy.) I only coded up exactly what I needed from the 1040 instructions book.
Then I went to a small now-forgotten company online two years, until the third year it turned out it couldn't handle ``income with respect to a decedent'' when that came up.
So I wound up with a large company online anyway.
I can't remember back far enough to when I might have actually done any genuine arithmetic myself unassisted.