How can I assign a group every single permission?

After the permissions rework, this is not possible without the use of an external permissions mod.

However, I'd like to note that this is NOT a good idea. In fact, this is why the ability to give the * permission was removed in the rework.

Some plugins will use permissions not as a way to enable commands but as a way to enable certain features, which you may not want. I could for instance make a permission like antiCheat.user.preventFlying that would stop anyone with the permission from flying. This would be desirable for ordinary users but potentially undesirable for admins.