So your position is that you can splice up actions?
So if you, say, cast a spell you can perform another action while you are resolving the spell? So cast Magic Missile, fire off one missile, do a bonus action, move about, then fire off the rest of the missiles?
You're making the game far too complicated.
No, I never said anything like that.
First of all, all bolts from Magic Missile strike their targets at the same time.
You create three glowing darts of magical force. Each dart hits a creature of your choice that you can see within range. A dart deals 1d4 + 1 force damage to its target. The darts all strike simultaneously, and you can direct them to hit one creature or several.
Based on that, it's entirely reasonable to move, cast Magic Missile, do a bonus action, and then move some more. Or, rearrange those 4 things into any order you like.
The perceived complexity here are the multiple attacks from Extra Attack. Given the special rule that says you can break up your movement and move between these attacks:
Moving Between Attacks
If you take an action that includes more than one weapon attack, you can break up your movement even further by moving between those attacks. For example, a fighter who can make two attacks with the Extra Attack feature and who has a speed of 25 feet could move 10 feet, make an attack, move 15 feet, and then attack again.
then I think it's a perfectly reasonable reading of the bonus action rules that you could also do a timing-independent bonus action like Misty Step or Healing Word between attacks granted via Extra Attack. Again, the bonus action rules say you get to decide when you take a timing-independent bonus action.
You choose when to take a bonus action during your turn, unless the bonus action's timing is specified, and anything that deprives you of your ability to take actions also prevents you from taking a bonus action.
Again, the timing of Misty Step and Healing Word is not specified, and thus you can do it any time you like (including between attacks from Extra Attack).
To beat a dead horse some more, Shield Master says you get the bonus action shove after you take the Attack action. JC has also clearly said that the intent is for Shield Master's slam to be a finishing move. Folks can decide to follow that intent or not at their table. The implication there is that the "you take the Attack action" part is resolved when all attacks from Extra Attack are taken, then the Shield Master trigger procs and you now have a bonus action (based on their "if X then Y" rules). Again, you can decide that your table will allow shove-slice-slice or slice-shove-slice or slice-slice-shove or whatever you want.
No matter what, bonus actions that don't have timing specified can be taken any time you like, and I don't think that makes the game any more complex (in fact I think it simplifies things -- you just have an extra thing you can do on your turn).