The Ready action states (emphasis mine):
[…] When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. […]
Thus, when you ready a spell you cast it before you release it; for further evidence of this there are the following questions:
- Can a readied spell be Counterspelled after it is cast, but before the trigger occurs?
- Can I deny a counterspell by readying my spell behind full cover?
To quote a few sections of the Sorcerer’s Metamagic feature (emphasis mine):
When you cast a spell that forces other creatures to make a saving throw, you can protect some of those creatures from the spell’s full force. To do so, you spend 1 sorcery point and choose a number of those creatures up to your Charisma modifier (minimum of one creature). A chosen creature automatically succeeds on its saving throw against the spell.
When you cast a spell that forces a creature to make a saving throw to resist its effects, you can spend 3 sorcery points to give one target of the spell disadvantage on its first saving throw made against the spell.
When you cast a spell that targets only one creature and doesn’t have a range of self, you can spend a number of sorcery points equal to the spell’s level to target a second creature in range with the same spell (1 sorcery point if the spell is a cantrip). […]
Thus, you do not wait until the spell forces a saving throw to use these features, instead you use them when you cast the spell; for further evidence of this there are the following questions:
- When must the wizard choose to overchannel?
- When do I have to declare that I want to twin my spell?
- Can a Sorcerer protect their friend from Web by using Careful Spell?
This all leaves me with some related questions (if they should each be separate questions then I can certainly split them up):
What happens when you try to use Careful Spell on a readied spell? If you can do this, when do you choose the affected targets?
What happens when you try to use Heightened Spell on a readied spell? If you can do this, when do you choose the affected targets?
What happens when you try to use Twinned Spell on a readied spell? If you can do this, when do you choose the affected targets?
Note that the question about the web spell above is rather similar to Careful Spell and Heightened Spell, though there is a different gap in time between the spell being cast and the saving throw being made; I’m unsure if that changes anything