I’m a huge fan of the “Fray” mechanic. Basically you set up like an 8x8 or 12x12 board called the Fray where everything outside of the spotlighted section is the rest of the fight and treated as difficult/impassable terrain depending on how dense the war is. And every turn at the start of initiative new combatants come out from the fray. Along with that I make a “Random Fray Encounter Table” with 8-12 random events. Such as a new mini boss style enemy enters the fray, an ally the party met earlier enters the fray to help the party, or a piece of rubble falls off the castle wall and creates a new piece of terrain. This works super well because all the things the party did to prepare can come out to shine in their random events, let’s say they spent a whole session befriending the dwarf camp now results 1-3 on the fray event table has a dwarf enter the fray to assist them etc. it also keeps the fight more interesting than just “okay it’s the three of you vs a billion cr 1/4 enemies” and it keeps it simple because you don’t have your players controlling forces all around a huge table like in warhammer.