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
"
...pushed the patch to master.
[1: 0e59541fa20]: 2023-11-04 14:56:36 +0100
Make FAQ on starting Emacs maximized portable
https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=0e59541fa203d7ecb6afcf0ce7805e87d41bdc65
" - https://lists.gnu.org/archive/html/emacs-devel/2023-11/msg00171.html