Awesome stuff!
The first thing to note is it's typically easier to ease into multiboxing by playing 5 of the same class. You went full Holy Trinity (1 tank / 3 dps / 1 heal) and that's probably the highest difficulty you'll run into.
To make it easier you'll want to find a way to have 1 button to press that does every role at 75%~ efficiency, and then have the other buttons for defensive or offense CD's, as well as movement keys.

In Legion I did run a Druid Tank / Druid Heals and 3 Enhancement shaman dps, and while it was fun I didn't feel like I was obtaining the level of efficiency I wanted, mostly the shaman were not performing to my standards.

Either way just learning everything, it adds an entire new path to this game and is very rewarding and enjoyable. Glad to hear your story.