Side-Based Action Initiative

I was reading (and loved) Prismatic Wasteland's take on action based initiative. I'm running some OSR play by post stuff soon and was thinking how to apply this to side-based initiative, which is what I'm planning on using. So here are two methods (caveat, these are not play-tested, just right off the top of my head, though I may try them in my upcoming games):

Side Based Action Initiative with Static Enemies

This is your typical side-based initiative which determines if PCs act before or after their opponents. folding in action types to this:

Unarmed attacks or weapon die of d6 or lower - Act before enemies

Weapon die of  d8 or higher, spellcasting, or multiple actions (if available in your system) - Act after enemies.

This leaves unspecified actions such as dashing, swapping items, manipulating objects, etc. I figure there is two ways to rule this: In Prismatic's system this is a d8, so it makes sense if these type of actions are a blanket act after enemies result. 

Alternatively, if using a system that employs skills or proficiencies, you could say that any character taking an action that uses one of their skills or proficiency acts before enemies, while any other skill or action acts after.

Side Based Action Initiative with Variable Enemies

This version of initiative acts like the above, but instead of one enemy turn there are two: faster enemies that act when faster PCs do, and slower ones that act when slower PCs do. This essentially creates quick and slow turns, similar to Shadow of the Demon Lord. In both cases, PCs act first in their designated phase. How it breaks down:

PC Unarmed attacks or weapon die of d6 or lower

Enemies with HD size of d8 or lower (Medium or smaller creatures)

PC Weapon die of  d8 or higher, spellcasting, multiple actions, or generic actions (as above)

Enemies with HD size of d10 or higher (large or larger creatures) 

Other Modifiers

So far pretty straightforward. What about modifying factors like bonuses or surprise? I think this is pretty easy to adjudicate. A side or PC with a bonus to initiative or some form of advantage can act first, regardless of action type. Similarly surprising an opponent allows you to act first. A monster with a speed-based bonus might act faster than its HD would normally allow.

If there is any kind of lair action or hazard, it happens at the top of the round. A multi-step hazard might act twice, especially if using the variable enemy type rule. 

Any type of condition resolution (saves, incapacitation) happens at the end of the round.

Overall, players will still have to think about their actions ahead of time, and can get a tactical advantage for quicker actions.

EDIT: I realized when implementing this for my Knave game that enemies all have one die size in old-school games. Here is the method I'm using in that case:

Decide your action at the top of the round. There are fast actions and slow actions making a fast turn and slow turn. All fast actions and fast enemies act on the first turn (PCs act first) then slow actions and slow enemies. 

Fast actions: Unarmed attacks, weapon attacks with damage dice of d6 or lower (including dual-wielding, as long as both are d6 or lower weapons), dashing, skills with which you are proficient. 

Slow actions: Weapon attacks with damage dice d8 or higher, casting a spell, multiple actions, all other actions.

Enemies act as their attack damage die or as makes sense for their other actions. d6 or lower attacks are fast actions. d8 or higher damage, multiple attacks, spellcasting are slow actions. Special abilities are typically slow but may be fast depending on the specific ability. 

Surprise or initiative bonus: If you gain the element of surprise you make a fast attack no matter the action / damage size. Enemies that gain surprise may act first in a given turn. 

If you cannot perform your planned action you can change it, but your action must take place on the slow turn.