Your claim that equality of Hilbert-Poincaré series implies equality of algebras is false.
Take $A=\mathbb C [X,Y]/(X^2)$
and $B=\mathbb C [X,Y]/(X^2+Y^2)$
with their quotient gradings.
We have $P_{A}(t)=P_{B}(t) =1+2t+2t^2+2t^3+2t^4+\ldots=\frac{1+t}{1-t}$ .
But $A$ and $B$ are not isomorphic as rings (and thus certainly not as graded algebras), since $A$ has a non-zero nilpotent element (the class of $X$), whereas $B$ is reduced i.e. has zero as only nilpotent element.
Generalization If you consider a homogeneous polynomial of degree $d$ in $n$ variables $f(X_1,\ldots,X_n)\in \mathbb C [X_1,\ldots,X_n]$, then the algebra $A(f)=\mathbb C [X_1,\ldots,X_n]/\lt f(X_1,\ldots,X_n)\gt$ has a Hilbert-Poincaré series independent of $f$, namely (combinatorists: please combine!)
$P(t)=\sum_{k\geq 0}\binom{k+n-1}{n-1}t^k-\sum_{k\geq d}\binom{k-d+n-1}{n-1}t^k$
However the rings $A(f)$ strongly depend on $f$. This is evident geometrically because the hypersurfaces $Z(f)=\{ x \in \mathbb C^n|f(x)=0\}$ vary a lot with $f$: they may be irreducible or not, smooth or not, etc. And so their rings of functions $A(f)$ differ a lot too and are not in general isomorphic for different $f\;$'s.
Edit I had overlooked that the OP wanted finite-dimensional examples. It is easy to modify the above examples by killing off all monomials of sufficiently high degree. Geometrically, this means looking at the intersection or the hypersurface $Z(f)$ with a sufficiently large infinitesimal neighbourhood of the origin. Let me do this explicitly for the first example and kill all monomials of degree $\geq 3$:
Put A'=\mathbb C [X,Y]/(X^2,XY^2,Y^3)=\mathbb C [x,y ]\; and B'=\mathbb C [U,V]/(U^2+V^2,U^3,V^3)=\mathbb C [u,v] .
Then P_{A'}(t)=P_{B'}(t) =1+2t+2t^2 \;.
However the algebras A' and $ B'$are not isomorphic: for example, their spaces of solutions of the equation $\xi^2=0$ have dimensions $3$ and $2$ respectively over $\mathbb C$. Namely, these spaces are:
Nil_2(A')=\mathbb C . x\oplus \mathbb C .xy \oplus \mathbb C .y^2 and Nil_2(B')=\mathbb C . u^2 \oplus \mathbb C .uv