You said it yourself, witch bolt "ends if you use your action to do anything else". Using your bonus action isn't using your action.
In 5e, it's just "action" not "standard" action, and as you also said "movement" actions aren't a thing. PH 189 says that "On your turn, you can move a distance up to your speed and take one action."
If you spend that one action doing not-witch bolt, the spell ends. Anything else you do with your move, bonus action, or reaction are immaterial—unless those actions violate one of the restrictions of the spell you already listed (like using your bonus action to misty step out of line of sight).
You can't take both Dash and Ready actions
Normally you don't "prepare a reaction". You take the Ready action instead. See Player's Handbook, page 193, "Actions in Combat"
Sometimes you want to get the jump on a foe or wait for
a particular circumstance before you act. To do so, you
can take the Ready action on your turn so that you can
act later in the round using your reaction.
Then you use you reaction (if still available) to actually perform the readied action when trigger occurs. You spend both your Action and your Reaction in this case. Therefore, you can't use your action to Dash after you declared a readied action, because it is already spent.
Unless you have an extra Action, or you're a Rogue
As notices by @NeilSlater, a Rogue has his Cunning Action feature, which allows him to use a bonus action for Dash. You can take the Ready action afterwards. However, this doesn't give you more movement than usual, since you just can Dash twice.
Also, you can't use "my turn ends" trigger
PH requires "perceivable circumstance" for a trigger:
First, you decide what perceivable circumstance will trigger your reaction.
Turns are game mechanics, they don't exist in the game world, hence, are not perceivable by the character, see Can you ready an action for "immediately before my next turn"?
You can move up to your speed using the Ready action
However, the question remains — what if you spent all your movement, and then take the Ready action to move 30ft more? Yes you can do this, providing your speed is 30 (or more) feet:
you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it
Ready action works like a "delayed Dash" in this case.
Best Answer
According to the PHB, page 189:
This means moving is not the same as taking an action. You may always move a distance up to your speed on your turn without taking any actions (barring special conditions — but let's not get into that!).
That you can always move on your turn without taking actions means there is no “move action”. Because why would there be, if you can always do this anyway?
The Dash Action
In the PHB, page 192, it says:
The Dash action is the closest thing to a “move action” in 5e, but it doesn't allow you to move. It allows you to increase your movement. You can always move, and taking the Dash action only helps you move faster.
Readying Movement
We've been talking about moving on your turn always being possible. But outside your turn, it is not so unless you take a specific action.
From the PHB page 193:
There is something called a reaction which is also available to you. For the purposes of this question, we don't need to dwell on what are the specifics of a reaction, though.
You may do the Ready action to do something on someone else's turn that is normally only possible on your turn, expending your reaction. This includes moving. (Note: this Ready action is not being used with a Dash action, it is being used to just move on another creature's turn.)