Is it possible to play Stellaris without the Launcher on Windows 10? I tried running the game directly from the stellaris.exe file, but the game just crashes at 30% on the loading screen. I also tried adding '-skiplauncher' but that didn't work either.

yes its possible under Windows 10, assuming you used the Galaxy install and not the offline then pressing the 'Launch Stellaris' icon should start with the launcher and pressing just Stellaris.exe [as long as Galaxy is NOT running in background] will fire up the game without the launcher

a crash at 30% will be the Paradox account sniffer, the save file location denies access i.e, dropbox | onedrive locks or just that your system is out of Ram

I don't use Galaxy, but it looks like Stellaris wasn't loading due to an antivirus issue. So I set the antivirus program to allow stellaris.exe to read the "Documents\Paradox Interactive\Stellaris" folder. The game finally loaded past the 30% mark.