After a detour to config includes and, again, several days/weeks of work implementing all edge cases and expected behaviors, true window maximize is ready and merged to main. Tricky cases like: windows requesting fullscreen and maximize after opening; windows failing to match the full maximized size; transparent windows with the niri border behind them.
https://yalter.github.io/niri/Fullscreen-and-Maximize.html
Give it a try! Ngl I mostly switched to maximize just because I'm too lazy to reach the keyboard for Mod+F.