C:\Program Files\Maxis\The Sims Online\TSOClient\Content there is a config file that allows you to change the GraphicsWidth and GraphicsHeight. I changed mine to my monitor recommended settings and that made it fullscreen, too, without changing Windowed to False.
I AM SOOOO EXCITED!!! Thank you sooo much for working on this! Where can I donate coffee money? You guys are amazing for doing this and I need you to stay awake until you finish it's pretty much the only way I can help... ;)