This.
I'm using an old mx518 [two extra mappable buttons + MMB for auto-run] and my laptop's keyboard.
Try setting up your keys in a way that you'll hit a few parts of your rotation easily and without much movement. Also, defensives must be easy to reach.
For example, my rotation is, typically, something like this: Shock [Q], Whither [3], Thrash [2], Shock [Q], Force Lightning [4], and then Assassinate [E] if they are low enough. I can use 3 fingers for this rotation, fairly easily and quickly.
Granted, I've been using the same basic keybinds [Q, E, R, T, F, V, C, X, 1, 2, 3, 4, + shift-versions, + mouse buttons] for years.
Just get used to it, play for a few days and make changes as you see fit. It's all about muscle memory.