Without the small-angle approximation the differential equation is
not the simple harmonic motion equation and the period is not
independent of the amplitude. The exact period can't be written in
simple closed form, but it can be written as an elliptic integral of
the first kind (which can be numerically approximated) or written as
a power series expansion. Short of that, various other
approximations are also available. Plot one or two of the
approximations you learn about below on the same graph as your
experimental data.