It can't be. You didn't declare release of energy as your later action. You declared casting a spell as your action.
Why isn't it?
You Ready a spell and you choose a trigger to release the energy. "release the energy" is the action you use with your reaction; if you choose to when the trigger occurs, and not if you don't choose to when the trigger occurs or if the trigger doesn't occur before your next turn.
This is clear in the rules.
Moreover you do not have to use your reaction if you Ready, but you do have to cast a spell if you Ready a spell (whether you use your reaction to release the energy or not).
The reaction is the action you would have taken on your turn.
No it isn't and this is clearly wrong. You take the reaction in response to a trigger which presumably has not even happened on your turn. The action you take on your turn is Ready.
From the PHB:
"You take the Ready action to wait for a particular circumstance before you act. To do so, you take this action on your turn, which lets you act by taking a Reaction before the start of your next turn.
First, you decide what perceivable circumstance will trigger your Reaction. Then, you choose the action you will take in response to that trigger ...
When the trigger occurs, you can either take your Reaction right after the trigger finishes or ignore the trigger."
Release energy is not your action, so it cannot be your reaction.
It is the action you take in response to the trigger. Why isn't it?
Why do you think this is not the action you take in response to the trigger when it clearly says it is in the description:
" .... but hold its energy, which you release with your Reaction when the trigger occurs."
Notice this wording is all in the same section:
....you choose the action you will take in response to that trigger .... which you release with your Reaction when the trigger occurs.
Seems pretty clear cut to me!
All that language above means is that when it's time to react, you act as you you would have on your turn.
NO. You take an action in response to the trigger, which is fundamentally different than acting on your turn.
You are not delaying your turn you are preparing to take a specific action in response to a trigger and you Ready a spell that action you are preparing to take MUST be to release the energy.