You are not Readying the Ready action and as I keep saying "Ready Action" is not a thing in 2024. I never said you are and you don't have to pick one of the actions off the table.
You cast the spell as part of Ready when you Ready a spell, the action you execute as a Reaction on the trigger is "release the energy"
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In this case you cast the spell on your turn as part of using the Ready action on your turn.
That last sentence is an interpretive proposal. It's not what the rules text actually says. What
the rules text says is
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. . . . When you Ready a spell, you cast it as normal (expending any resources used to cast it) but hold its energy, which you release with your Reaction when the trigger occurs.
Here's an alternative interpretation of that text: what one
readies is the action that is performed as a Reaction when the trigger occurs - in this case, the release of energy. That is consistent with "wait[ing] for a particular circumstance before you act." Thus, when, as your action for your turn, you take the Ready action in order to Ready a spell, you ultimately perform 3 actions:
*The Ready action, which is one of the "main actions" in the game;
*Casting the spell as normal;
*Acting as a reaction, releasing the energy that you have been holding.
What do we make of that second action (
casting the spell as normal)? The rest of the rules text for Ready doesn't help, because the rest of that text works on the basis that there are only 2 actions: Ready, and the readied action performed as a Reaction. Readying a spell is a departure from this structure.
The rules for the action economy don't help, because
they say that you take only one action on your turn. So we're talking about an exception from those rules.
There are some rules elements that use the language of "as part of" - eg
Divine Intervention says "As a Magic action, choose any Cleric spell of level 5 or lower that doesn’t require a Reaction to cast. As part of the same action, you cast that spell". But that language isn't used for the Ready action.
In my view, the clearest way of answering this question is to treat "casting the spell as normal" as
performing the Magic action. That is consistent with
the Casting Time rules:
Most spells require the Magic action to cast, but some spells require a Bonus Action, a Reaction, or 1 minute or more. A spell’s Casting Time entry specifies which of those is required.
The second sentence in the Casting Time rules says that a spell's casting time entry specifies
which of those - the Magic action, a Bonus Action, a Reaction, or 1+ minutes - is required. In other words, it treats the list as exclusive, not inclusive.
A spell, if it is to be readied, "must have a casting time of an action". From the exclusivity of the list of casting times, that means the Magic action.
Hence why, in my view, the most straightforward way of making sense of
what action is performed when a character who is readying a spell "casts the spell as normal" is that the character performs the Magic action.