By RAW actions have a duration. That duration is 1 action. A duration of 1 action is a length of time that is greater than instantaneous, because we know that spells that are bonus actions are exceptionally swift.
Okay, I'll go with the duration of an action is an action. Instantaneous isn't the same as an action, though, so actions and instantaneous aren't even in the same category.
Actions do not have a duration. I'm really not sure how else I can say this. You (well, lots of others as well) have decided to add duration to actions when they have none. Not have a duration doesn't mean not taking some fictional time, it just means that a defined duration isn't part of actions at all. Discussing the duration of actions is declaring that you've trumped four sevens with your eight of spades while playing poker. It's nonsense.
You've just declared that the Cast a Spell action, which takes 1 action worth of time, takes zero time and that bonus spells take even less than zero.
Nope, you're not listening. You're hearing me say "actions have no duration" and hearing "the duration of an action is zero." These are not the same thing, at all. I'm saying that actions do not have the quality duration. I don't care what length duration because actions do not even have this quality.
That makes no sense. Each action has a duration of 1 action. If it takes more rounds to do it, it just means that you have to use multiple Cast Actions on multiple turns, each taking 1 action in length, in order to complete the spell.
Exactly right. I wonder if you'll get it, though.
It's RAW. 1 action is in fact a unit of time in combat. Actions take 1 action to complete. That's the rules.
Exactly right. I wonder if you'll get it, though. Hint, this time: instantaneous is completely nonsense when talking about actions. The have a duration of themselves. This is a tautology, which is why the rules do not actually say, anywhere, that the duration of an action is an action. They just talk about actions. You've come up with "actions take 1 action to complete" and declared it RAW. Trivially, it is, because it's a tautology.
They take 1 action. Bonus actions are swifter than that. They didn't write in that actions are 2.4 seconds and bonus actions are .6, if that's what you are looking for. But they absolutely write actions as taking time and bonus actions as taking much less time
Are they? Are all bonus actions swifter? You can, for instance, use the Dash action as a bonus action if you are a rogue with the Cunning Action class ability. Dash take exactly Dash long to complete (another tautology), whether it's an Action or a Bonus Action.
Some spells are listed as swift and so are bonus actions, but this doesn't travel, or really mean much. Bonus actions take 1 bonus action to complete, to reuse the tautology.
How long an action takes isn't anything the rules actually discuss or even care about. On your turn you have an action and a move, and maybe a bonus action. It all fits in your turn, regardless of which specific ones you pick, and your turn always takes up the same amount of time. Actions just do not even care about duration -- it's utterly nonsensical to discuss duration with regard to actions.