Not sure exactly what's going on, but I have Sodium 0.6.0 Beta 2 for 1.21.1 for NeoForge and Iris 1.8 Beta 4 for 1.21.1 Neoforge loaded. For whatever reason, when I go and play the game, I get stuck on the Mojang screen. Weirder enough, after a few seconds, I can hear the main menu music which makes me feel like the game actually loaded but is stuck doing something weird.
Edit, in fact I can click on the "main menu" buttons through the Mojang loading screen. So it clearly is loaded correctly, but something is causing the Mojang Loading screen to be stuck.