$$f(x)=2mx^{2m+1}-(2m+1)x^{2m}+1$$
In terms of reals roots, there is a double one at $x=1$ and another which is negative. If $x_{(m)}$ is this root, we have
$$-1 \lt x_{(m)} \leq -\frac 1 2$$ To work for this root is not very simple with $f(x)$. What I found more convenient is to let $t=-x$ and search for the zero of function
$$g(t)=\log \left(t^{2 m} (1+2 m (t+1))\right)$$
Interesting (but I do not see how to use it) is to notice that the problem is "hust" the inverse of
$$2m= \frac{1}{\log (t)}\,W_{-1}\left(\frac{ \log (t)}{t+1}\,t^{\frac{1}{t+1}}\right)-\frac{1}{t+1}$$ where $W(.)$ is Lambert function.
Now, to have an estimate of $t_{(m)}$, we can make one single iteration of a Newton-like method of order $k$ starting with
$$t_0=\frac 12+\frac{3 m+1}{6 m (2 m+1)}\log \left(\frac{4^m}{3 m+1}\right)$$