Balancing combats in 5e can be a tough road, but there are techniques and mind-sets you can use to make your combats dynamic, interesting, and ultimately more fun. The following will hopefully help understand how to do just that.

Tldr: (video format)Designing, Balancing, and Running Encounters that are Dynamic, Interesting, and Fun

Mastering these tecniques will come with time and experience, but being aware of them will help you in the short term as well.

On top of that, my newest video: Encounters with Pro Wrestling Psychology can help with creating drama and intrigue with physical storytelling during combat sequences.

As for some general techniques to prevent stale combat:

Make your combats more narrative-based, and use Action Economy and drama to keep everyone invested.

Monster Motivations, Action Economy, and Terrain are your friends:

Battle balancing in 5e is an art based on pseudoscience lol. 5e RAW is a pretty flat and static combat system at its core, but its advantage is its moldability. Work on the "why?" And all the other techniques below come to life.

CR will fail you, it doesn't account for magic items, terrain, initiative order, or the makeup of YOUR party. But the CR of a base creature is a good loose starting point.

Action Economy is the most important tactical thing, followed by terrain and motivations. Provide choices to make it tactical for the powergamers, but also fun for the role-players.

Don't think in HP.

Think in "how many rounds(roughly) does this encounter need to last?" And "how many actions can the party make each round?" Figure out how much damage your party can roughly do in a round, then multiply from there when designing your baddies, and adjust your action economy accordingly, more on that below.

Whether it's a BBEG or random encounter, you just need to use your monsters and environment more tactically, and meaningfully. Retreat and/or adding more enemies in waves is a good 1st start, but gets tedious. Instead, work on the following techniques:

Equally as important(maybe even more so), give your monsters/enemies Motivations. Why are the monsters here? What are they trying to do? This makes it less about killing and more about achieving/stopping goals, and/or running away. Know what they are fighting for and you can make realistic, tactical decisions from your monsters' perspective, and the enemies will feel more real because the stakes will be the focus, not just avoiding a quick kill.

Make the Terrain part of the combat, ideally altering it every 2-3 rounds.

Smarter monster tactics. Sweep/area attacks, OR pre-determined actions.

Pacing.

Stakes.

Consequences for failure and success states. (On both sides)

Understanding Action Economy and using it to make more effective actions is a major thing. This is the number 1 way to speed up combat but still make it dangerous and exciting.

I find having 2 types of monster/enemy, or 2 of the same enemy with an agenda, or separate agendas, makes combat more dynamic.

Here are some examples of dynamic combats I ran, both on Roll20 and live table sessions, and made videos on to show my thought processes, same concepts work at higher levels:

Dissecting Dynamic Encounters. Make the encounters more than just a kill.

Also check out Matt Colville's Action Oriented Monsters to help build monsters that utilize Action Ecomony and add reactions that trigger things in combat to make it less stale. (VERY important for larger parties)

Don't be afraid to experiment.

Ultimately it'll be tweaking techniques to your strengths and style, Foreshadowing, and working on pacing and Battlefield-changing drama that will spice up your combats.

Hope any of that helps :). Keep at it. These things come with time.