Apologies for joining late, and perhaps you have already tried and
discounted this approach, but: Have you tried adding:

(push '(fullscreen . maximized) default-frame-alist)


or perhaps

(push '(fullscreen . maximized) initial-frame-alist)


to early-init? I put the first above line in ~/.emacs.d/early-init.el, and
got an initially maximized window under an odd Wayland+XWayland system as
well as Win 11. I don't have reasonable access to either a more typical
X11, Wayland, or macOS system right now.

The key is to put it in early-init.el, which is consulted before emacs
makes any windows in any window system. Also worth noting: if you end up
with conflicting elisp settings and X resources, there are a variety of
ways for the latter to silently override the former. You can check this
with "xrdb -query" (you might want to run this in a shell buffer or pipe it
to a pager).

Hope this helps,
~Chad