What are the basic limitations for using a Reaction to move? Does movement used on your Turn matter?

Question

Please help my players and myself who are having trouble understanding what movement is allowed when using Reactions to move during and after each Turn in a Round.

In the player's handbook in "Actions in Combat" it says under the heading "Ready","(...)you choose to move up to your speed in response to it."("it" being the trigger for the Reaction)...

Does this mean even if you've used up all of your movement for the Turn, you would still be able to move your Speed during your Reaction? I am very confused by many parts of this and what it means for combat gameplay.

1. Is this possible assuming you have a Speed of 30ft?

   On your turn:

   • Move 30ft
   • Action used to Dash
   • Move 30ft
   • Reaction prepared "I ready my Reaction to move 30ft forward immediately after my turn ends"
   • End your turn
   • Reaction triggered, you move 30ft forward

2. Is this possible assuming you have a Speed of 30ft?

   On your turn:

   • Reaction prepared "I ready my Reaction to move 30ft forward after I have moved 5ft"
   • Move 5ft
   • Reaction triggered, you move 30ft forward
   • Move 25ft
   • Action used to Dash
   • Move 30ft

3. Is it conductive & beneficial for good gameplay to have players

   • readying their Reactions to move with leftover movement every Turn?

   • readying their Reaction every Turn so that they can move to get AoO against opponents who decide to move?(related→bottom link)

   • readying their Reaction every Turn against potential readied enemy Reactions?

Similar questions that do not discuss movement/Speed limitations:

• How is a Ready action handled if it triggers on your own turn?
• In 3.5 DnD, readying a move action to go adjacent to an enemy provoking AoO

RAW & RAI are very much appreciated, but "Rules for Better Gameplay" will be the most praised. I am looking specifically for help that will lead to better gameplay movement mechanics regarding Reactions not only so that I may better understand & teach the rules to others, but also to ensure quality gameplay for my players. (though a better understanding of the rules and where they can be found in the rulebooks will help everyone greatly)

Answer 1

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.

Answer 2

Movement on your turn doesn't matter for moving as a reaction

To answer your main question, Movement you spend on your turn doesn't affect how far you can move as a reaction. From the PHB:

First, you decide what perceivable circumstance will trigger your reaction. Then, 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.

It doesn't matter how much you moved prior to the Readied Action triggering, you can move up to your current speed (which may be different than what your speed was when you Readied the Action, if you got hit by certain spells for example).

Readying an Action or Movement costs an Action

The scenarios you proposed don't work in general because 'Ready' is an action by itself (and you only get 1/turn). Other actions that compete for the Action resource are Attack, Dash, Cast a Spell, etc.

How this works is:

1. You move (up to your speed)
2. You use your Action to Ready either an action (like Attack) or to Move (Set some perceivable trigger, ending turn isn't possible because you can't perceive turns in-game).
3. You end your turn
4. If whatever you set as a trigger happens you can decide to use your Reaction to do whatever you set up earlier, or to ignore the trigger. If you ignore the trigger your Readied Action can be triggered again. If you take the Action then you do whatever you decided earlier as soon as the trigger completes.

Rogues and Fighters can Dash and Ready on the same turn

Rogues get Cunning Action which lets them use Dash (among other things) as a Bonus action, so they can use their Bonus to Dash and their Action to Ready.

Fighters get Action Surge which lets them take 2 Actions on a turn, so they can both Dash and Ready.

Your third question

In regard to your third question, none of those are very good ideas. In general it is better to be proactive, especially if you can damage and take out an enemy entirely. Enemies can't hurt you if they are dead (except for Undead...). Readied Actions are useful for specific scenarios, mostly when there is nothing productive a PC can do on their turn but there will probably be something useful before the start of their next.