Your suggestion with Life sounds more like generalizing the Busy Beaver function for a fixed universal Turing machine to cellular automata. To generalize the usual Busy Beaver function (that quantifies over Turing machines with a fixed number of states), we should look at all cellular automata of some "size" and see when
they "halt".

At least myself and Guillaume Theyssier have independently looked at nilpotency on all configurations as a possible way to get a nice Busy Beaver function like that. I discussed this as a kind of preamble to a talk of mine about nilpotency http://www.villesalo.com/notes/SANiCASlides.pdf
(2 states, radius $\leq$ 3)
and Guillaume Theyssier has done the same at
http://theyssier.perso.math.cnrs.fr/castor/
(3 states, radius 1).

The definition is: Write $\mathrm{CA}_{m,n}$ for the set of cellular automata
$f : A^{\mathbb{Z}} \to A^{\mathbb{Z}}$ where $A = \{1,2,...,m\}$ and $f$ admits
a neighborhood of $n$ consecutive cells meaning $f(x)_i = F(x|_{\{i,i+1,...,i+n-1\}})$ for some function $F : A^n \to A$ (we can w.l.o.g. assume the neighborhood is on the right, as composing by a shift will not affect nilpotency). Now, define
$$\mathrm{MND}_{m,n} = \max\{k \;|\; \exists f \in \mathrm{CA}_{m,n}: f^{k-1}(A^{\mathbb{Z}}) \neq \{0^{\mathbb{Z}}\} \wedge f^k(A^{\mathbb{Z}}) = f^{k+1}(A^{\mathbb{Z}}) = \{0^{\mathbb{Z}}\}\}$$
i.e. $\mathrm{MND}_{m,n}$ is the maximal $k$ such that some CA with neighborhood size $n$ on an alphabet of size $m$ is nilpotent at exactly step $k$.

I think this is the right candidate, because nilpotency is one of the most important undecidable problems in the theory of cellular automata.

I give the following values in my talk (with a different convention): $\mathrm{MND}(2,3) = 2$, and I give an example that does not die after $7$ steps and seems to be nilpotent, suggesting $\mathrm{MND}(2,7) \geq 7$. I have since proved (sloppily and undocumentedly) that that CA is nilpotent after $9$ steps, so indeed $\mathrm{MND}(2,7) \geq 7$.

Theyssier's record is $\mathrm{MND}(3,3) \geq 19$, for these examples we also don't know what the actual exact nilpotency time is.

Florian Bridoux showed $\mathrm{MND}((4q)^2,3) > q^q$ after a talk of mine, and also showed that $\mathrm{MND}'(2,3)$ is infinite for the variant where the neighborhood need not be contiguous.

By undecidability of nilpotency and standard embedding arguments, $\mathrm{MND}(m,n)$ grows as a busy beaver function in both variables (assuming $m \geq 2$), i.e. faster than any total recursive function.

I don't know enough about Life to answer your actual question.