Is it not the case that you can (for an action that is not instantaneous, like dodge) divide your action with pretty much anything that can be done on your turn? Movement, flourish, interaction, communication, Reaction... all of those can be done between attacks in an Attack Action, certainly. It seems that you are singling out bonus actions as the only thing that cannot divide attacks within an Attack Action, and so if there is a rule that states an Action to be indivisible by Bonus Actions (when it is divisible by, as far as I can see, everything else) I would expect that rule to be clearly and unequivocally written.
Actually, flourish/speaking/object interaction are specifically called out:
Other Activity on Your Turn
Your Turn can include a variety of flourishes that require neither your action nor your move.
You can communicate however you are able, through brief utterances and gestures, as you take Your Turn.
You can also interact with one object or feature of The Environment for free, during either your move or your action. For example, you could open a door during your move as you stride toward a foe, or you could draw your weapon as part of the same action you use to Attack.
If you want to interact with a second object, you need to use your action. Some Magic Items and other Special Objects always require an action to use, as stated in their descriptions.
The GM might require you to use an action for any of these activities when it needs Special care or when it presents an unusual obstacle. For instance, the GM could reasonably expect you to use an action to open a stuck door or turn a crank to lower a drawbridge.
So, yes, I am singling out Bonus Actions, because, besides taking a move and an Action, that's the only other thing in a turn.
Look, it's pretty simple. For the vast majority of Actions, we can all agree they encompass single, specific actions that aren't divisible. Dash, Dodge, Cast a Spell, even (although that might be arguable with things like EB or scorching ray). The issue here is the Attack Action, and really then only after you get multiple attacks. But, the thing is, Extra Attacks is a modifier to the Attack action itself, not a new extended Attack action that becomes divisible. It modifies 'one attack' to 'more than one attack'. That's it.
Now, there's another form of argument that is declaring an Attack action sets a game term flag that immediately enables all things that would key off of that. But, this is entirely devoid of any indication in the rules and is a form of reading into the rules an increased specificity of flow that's absent from both the RAW and the RAI. This edition intends less gamey reading, so assuming that declaring an attack action is separate from taking the Attack action, or that taking the Attack action is a concrete substep, isn't mandated by the rules at all. Instead, on your turn, you move and take one action. What you do when you take an action is listed under that action -- that's what happens when you take that action. It isn't a new stack of things that gets resolved LIFO, it is a discrete unit of game that resolves entirely before moving on, outside of explicit exceptions. Like talking, object interactions, and spending your move.
I read the line of taking your bonus action when you want as a reference to the timing -- you can take an untimed bonus before or after your action. Taking it during would require exceptional reading.
