What are some creative ways to balance action economy?

Hello, Im a rookie dm on my first year being the storyteller of a few different groups of adventurers.

So far, the only way Ive been able to balance somewhat balance economy is by giving boss characters multiple phases per turn, they dont always attack on each phase tho.

Think about it this way,

Phase 1 - Initiative 16
Phase 2- Initiative 8

Sometimes they will shove a player, create an environment hazard, kick someone or produce an obstacle, then in their second phase they will actually attack.

Ive found it hard to balance out action economy for bosses specially after level 5 when many classes get a second attack, so a party can have some 8-10 main actions as a team while bosses get 1.

Im trying to evade the urge of providing them legendary actions as I believe some of those are sometimes overtuned.

I appreciate any responses given, thanks!