I know there's a lot people don't like about 4e, personally, I think it was a solid enough game just in a completely different playstyle to what people associated with D&D and should have been its own thing. Anyway, the thing I want to point to here is the minion idea from that game.
There were classes of enemies that were called minions. They had 1 hp no matter what level they were. So a 15th level minion would have appropriate to-hit bonuses and stuff to be a threat, though even there they tended to do low-damage and/or low chances to hit, but would have 1 HP. Some would have minor special abilities to add flare to the battle, but again, they were mostly minor threats that just had to be maintained to a low number.
This allows for larger battles that still feel like you're doing something as you choose between clearing out the minions and targeting the major threats. Like the fight against Dror Ragzlin would be a good place to add a few 1 HP goblins to clutter the field up.
You might also consider reinforcements. This is something my brother does in his larger fights. He basically does a lair-action to have new enemies added to siege battles. Usually these are weak enemies that can be taken out easily enough, but occasionally a larger threat is rolled up. In one case, this was an infinite situation where we had gates magicially spawning various types of zombies at us one after another while we tried to solve a puzzle.
Having enemies that can be easily cleared with cantrips and basic attacks....or cleared on mass by things like Thunderwave or Shatter, might be a good way to get more epic battles out of things. Or a different flavor of epic battle anyway.
A 4 v 4 battle of elites is fun. A 4 v 1+bodyguards is fun in another. And 4 v horde yet another. But you can't do a 4 v horde very well if the entire horde are HP sponges.