Consider that all of the following things are connected to a turn:
- Bonus Actions
- Object Interactions
- Events that happen automatically during a turn
- Possibly other things I may have forgotten.
If all you need is more attacks, then that means you need to fix everything that is not that attack. Some of the things that need to be fixed seem easy, because they are found on the creature’s sheet. For example, to fix the creature’s movement you give it less speed (as you yourself plan to do).
However, not everything the creature interacts with is found on its own sheet! How many things exist that interact with movement alone? More than I could count, and every single one has a potential of creating unintended consequences.
For example lets say a creature has a speed of 10 feet so that over the course of its 4 turns it can move 40 feet and someone hits this creature with a ray of frost, which reduces its speed by 10 feet. Now instead of being able to move 30 feet per round, the creature can move 0 feet per round. What do you do now that you have spotted this unintended consequence?
Do you accept that ray of frost is OP against this creature?
Do you create a dedicated ruling for ray of frost?
Do you create a generic ruling for speed reductions (thereby risking new unintended consequences)?
Moreover, some of the unintended consequences can be very nuanced, hard to spot, and/or hard to quantify. For example, a creature with multiple turns can use its strongest at-will attack over and over again, which is boring; whereas the design of the Multiattack action (and legendary actions and lair actions) often forces a creature to use all its attacks, which adds variety even though some of those attacks are weaker.
Now scale up this problem to every feature, spell, ability, trait, magic item, etc that interacts with movement, actions, bonus actions, reactions, object interactions, and/or events that happen automatically during a turn.
You don’t have the time to research all these interactions, find the unintended consequences, and devise all the necessary rulings. Therefore, you will have to improvise on the spot when you encounter an unintended consequence, you will have to document those improvised rulings so that you can be consistent the next time, and your players will have to deal with an increasingly large list of improvised rulings.
You should not put yourself and your players through all this trouble when your problem already has well documented solutions:
- Use legendary actions
- Use lair actions
- Use more enemies