Elimination of variables with Groebner basis

I am trying to solve equations 1 and 3 shown below:

For nmax = 2, I was able to solve for the value of z but higher than that Mathematica keeps on running forever and also I believe at some nmax sum must converge so I am trying to get that value of z.


,

Could someone simplify the code or suggest some other way to solve for the value of z?

I have attached Mathematica file.

groebner_basis.nb (13.4 KB)

This problem has been solved.