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