When I start 1.21.6 with the built-in default connected textures resource pack enabled, my machine will freeze until I put it in and out of sleep mode. I don't know if it's an issue with my machine or not, but I've never had such an issue with any game before and it only happens with that resource pack enabled; disabling it causes the game to start fine.

This is definitely not a typical issue. Enabling the default pack will put more textures into the block atlas and use more VRAM. You could try updating your graphics drivers.

I never did figure out what the issue was. Most likely it is my machine, but I just wanted to bring it up in case it was an issue you needed to fix on your end.