  The 21-cm line emitted by neutral hydrogen is the most promising probe of the
Epoch of Reionization (EoR). Multiple radio interferometric instruments are on
the cusp of detecting its power spectrum. It is therefore essential to deliver
robust theoretical predictions, enabling sound inference of the coeval Universe
properties. The nature of this signal traditionally required the modelling of
$\mathcal{O}(10^{7-8} \, {\rm Mpc}^3)$ volumes to suppress the impact of cosmic
variance. However, the recently-proposed Fixed & Paired (F&P) approach uses
carefully-crafted simulation pairs to achieve equal results in smaller volumes.
In this work, we thoroughly test the applicability of and improvement granted
by this technique to different observables of the 21-cm signal from the EoR. We
employ radiation-magneto-hydrodynamics simulations to ensure the most realistic
physical description of this epoch, greatly improving over previous studies
using a semi-numerical approach without accurate galaxy formation physics and
radiative transfer. We estimate the statistical improvement granted by the F&P
technique on predictions of the skewness, power spectrum, bispectrum and
ionized regions size distribution of the 21-cm signal at redshift $7 \leq z
\leq 10$ (corresponding to $\geq80\%$ of the gas being neutral). We find that
the effective volume of F&P simulations is at least 3.5 times larger than
traditional simulations. This directly translates into an equal improvement in
the computational cost (in terms of time and memory). Finally, we confirm that
a combination of different observables like skewness, power spectrum and
bispectrum across different redshifts can be utilised to maximise the
improvement.
