  We provide analytical and numerical insights into the phase diagram and other
properties of the extended Kitaev-Heisenberg model on the honeycomb lattice in
the {\it easy-plane} limit, in which interactions are only between spin
components that belong to the plane of magnetic ions. This parameter subspace
allows for a much-needed systematic {\it quantitative} investigation of spin
excitations in the ordered phases and of their generic features. Specifically,
we demonstrate that in this limit one can consistently take into account magnon
interactions in both zero-field zigzag and field-polarized phases. For the
nominally polarized phase, we propose a regularization of the unphysical
divergences that occur at the critical field and are plaguing the
$1/S$-approximation in this class of models. For the explored parameter
subspace, all symmetry-allowed terms of the standard parametrization of the
extended Kitaev-Heisenberg model, such as $K$, $J$, and $\Gamma$, are
significant, making the offered consideration relevant to a much wider
parameter space. The dynamical structure factor near paramagnetic critical
point illustrates this relevance by showing features that are reminiscent of
the ones observed in $\alpha$-RuCl$_3$, underscoring that they are not unique
and should be common to a wide range of parameters of the model and, by an
extension, to other materials.
