I wrote up something quick after being inspired by your post:

Step 1. Players roll initiative. Adds initiative bonus. Step 1.5. Dm rolls initiative for monsters. Step 2. Players are lumped into three Groups: those who score 20+, 10+ and below 10. Step 2.5. Monsters are grouped similarly. Step 3. Groups who CRIT on initiative may act after anyone who gained a surprise round but before anyone acts normally. They may use this free round to move and use a bonus action. If there are more monsters who CRIT then they go first. If there's a tie 1 roll per side will decide it. Step 4. By number of each individual in each group, decide which groups outnumber the other group on the player’s and DM’s side in the same categories (under 20, under 10.) The outnumbering group goes first always, regardless of changes due to additions and subtractions. Step 5. The players and monsters of 20+ take turns in whatever order they like being able to intermix their turns,actions and movement, sharing the turn as a group as if controlling 1 unit with multiple actions independent of each person’s turn. For example John moves up to monster X and attacks, miss, player Julie moves up to Monster X and uses her action to Help John (a fighter with two attacks) who makes a second attack with advantage and hits.
Step 6. Players and monsters in group 10+ can take their turn in any order. Step 7. Under 10’s go by highest initiative in order. Roll decide ties of course. Step 8. Repeat from step 5.

I'd love to get some feedback that could streamline it and make it simpler.

I think instead of arbitrarily making players go round the board while for example, the Giant can act first and then the goblins swarm(they don't sit around a table), can be unfair and leads to problems with a lack of teamwork. My way allows for creative uses of spells, synergy and smart thinking for both the Pc's and especially the DM.

'The goblin expertly scales up the giant's back, giving you the impression it has done this many times before. It saddles up on the big guy's shoulder pulling his huge ear in close as he begins to shout commands to the dumb-brute. The giant's dull eyes become clearer and focus on you as he swings his massive club' - he has advantage due to the goblin using the help action.