How to account for sample rate in a FIR Digital Filter?
The sampling frequency is hidden in your variable $\theta$. The angle $\theta$ determines the frequency of the notch relative to the sampling frequency, i.e.

$$\theta=2\pi\frac{f}{f_s}\tag{1}$$

where $f$ is the notch frequency in Hertz, and $f_s$ is the sampling frequency (in Hertz). So if you want to change the sampling frequency while leaving the notch frequency unchanged, you need to change the angle $\theta$ according to Eq. (1).

The gain of your filter depends on the normalized frequency variable $\theta$. At DC the gain is $2(1-\cos\theta)$, and at Nyquist you get a gain of $2(1+\cos\theta)$. By normalization you can keep one of those two gain factors normalized to $1$, but never both. If you want a gain of $1$ at DC and at Nyquist you need an IIR filter or a higher order FIR filter.