I’m not going to respond point by point, but I have been generally thinking of something similar to this for a while, possibly with the added complexity of something like speed factors, not to the granularity of 1e, but in the “Immediate actions go first, then some system for things that take movement (or otherwise eat up time).” Initiative boils down to an instant: you might get the jump on the goblin archer, but if you’re trying to run across the field to hit him with a sword before he puts an arrow in you, it won’t work.
It’s tied into a houserule I’m tinkering with to make initiative “side-based” (d6, high side acts first) but staggered. What do I mean by “staggered?” PC(s), monster they attack, PC(s), monster, and so on.
Basically, if two or more on either side tag-team someone, the initiative winning side goes first, then we resolve their opponent’s turn, then move to the next group.
That sparked the questions: “Shouldn’t archers who are ready get to shoot before people can move? How long should spells take to cast, etc.” And that’s why the houserule is still in development.
Side note on your d6 thing is you could get big dice that could be seen from across the table. I might think about having people roll a d6, but using a large d12 to represent your turn - if I add my speed idea to it (“You want to run over to that goblin? Okay, moving across the room takes 3 segments, so increase your die by 3 and we’ll resolve your attack then. Next person.”)
Clearly still a work in progress.