Not sure what you’re talking about #1 slot, it doesn’t have some special exemption. The number slot has nothing to do with GCD reset. That’s something that applies to any slot that isn’t currently on cool down.
Read Loraen’s guide here that specifically talks about spell line ups. You will get a general feel for what you should be using. He even has a few YouTube videos linked of some solo battles I recommend you watch them you’ll get a feel for how switching it done at the highest level.
https://wiki.project1999.com/Loraen%27s_Enchanter_Guide