Wheeler de Witt equation for FLRW cosmology, directly from general WdW equation

Quantum geometrodynamical WdW equation is given by:

$
hat{H}psi=(frac{1}{2sqrt{h}}G_{abcd}hat{Pi}^{ab}hat{Pi}^{cd} -sqrt{h}hat{R}^{(3)})psi=0
$

for generic ADM splitted metric:
$ds^2=-N^2dt^2+h_{ab}(dx^a+N^adt)(dx^b+N^bdt)$.

With mommentum operator:
$hat{Pi}^{ab}=-ifrac{delta}{delta h_{ab}}.
$

Lets consider FLRW spacetime: $ds^2=-N(t)^2 dt^2+a(t)(dx^2+dy^2+dz^2)$, shift vector vanish ($N^i=0$) and $N=1$ and $h_{ab}=a(t)^2delta_{ab}$ (Kronecker delta/Euclidean 3d metric).

My question regards how to impose WdW equation for FRW cosmology directly from generic version – in literature which i found, cosmological WdW equation is deduced from Lagrangian – one obtains conjugate momenta w.r.t. $(a,N,N^i)$, then Hamiltonian constraint is deduced and to obtain WdW equation conjugate momenta w.r.t scale factor $a(t)$ is promoted to quantum operator $=-ifrac
{delta}{delta a}$ (standard quantisation procedure).
But how to obtain WdW equation for this metric using directly general version ? What happens with contractions of the metric $h$ when $-ifrac{delta}{delta h_{ab}} rightarrow
-ifrac
{delta}{delta a}$, which should be the case for the FRW cosmology (from my naive thinking, im left with supermetric $G_{abcd}$ with 4 free indices, while this part of WdW equation should be $sim frac
{1}{12a}frac{delta^2}{delta a^ 2}Psi$)?

Examples of references: https://arxiv.org/abs/1404.4195,