Infill has massively diminishing returns. I don’t think 100% is required. Usually it’s much better to increase the number outer layers.
That said, depending on the cost scaling, the possibly positive effects of extra mass, and how much I want to avoid a second attempt, massive objects can make some sense.

PLA becomes brittle with moisture (from your hands and/or air). I would recomment PETG / ASA / ABS (ascending order).

Yes, basically all CAD and slicers (3d-printer software) can mirror. PrusaSlicer for example can mirror and then save to stl again.