A brief disclaimer that I've not tried this in 3.5, but I see no reason it couldn't work.

If I'm understanding your question correctly, your issue stems from the players dropping enemies too quickly and are at constant risk of being dropped too quickly themselves.

My recommendation is take a note from 5th edition and use their death and dying system instead. 5e is swingy with damage in a manner similar to 3.5, but the swings are tempered by a more forgiving death system than 3.5 uses. RAW for 3.5 are -10 equals dead, which can be just incredibly easy to hit with some enemies after level 5.

5e works differently. There's no negative numbers, you're knocked to 0 then you're unconscious and dying. As a result, healers can cast a low level heal spell to get someone back in the fight. This can work for both sides of the fight so although your big monster was dropped in one round, they can be brought back in by a low level heal spell.

It's up to you if you want to also bring over 5e's death saving throws or if you just want to allow players to utilize 3.5's Coup de Grace rules to finish off downed enemies.

Again, I've not used this in 3.5, but when I fully experienced it in 5e I found that it can be a bit like playing whack a mole. This can be a good or bad thing, depending on what you're trying to do.

This won't stop players and enemies from dropping quickly, but it will reduce the lethality of combat somewhat and easily allow both sides to continue fighting.

A brief disclaimer that I've not tried this in 3.5, but I see no reason it couldn't work.

If I'm understanding your question correctly, your issue stems from the players dropping enemies too quickly and are at constant risk of being dropped too quickly themselves.

My recommendation is take a note from 5th edition and use their death and dying system instead. 5e is swingy with damage in a manner similar to 3.5, but the swings are tempered by a more forgiving death system than 3.5 uses. RAW for 3.5 are -10 equals dead, which can be just incredibly easy to hit with some enemies after level 5.

5e works differently. There's no negative numbers, you're knocked to 0 (if you go negative you're still at 0) then you're unconscious and dying. As a result, healers can cast a low level heal spell to get someone back in the fight. This can work for both sides of the fight so although your big monster was dropped in one round, they can be brought back in by a low level heal spell.

It's up to you if you want to also bring over 5e's death saving throws or if you just want to allow players to utilize 3.5's Coup de Grace rules to finish off downed enemies.

Again, I've not used this in 3.5, but when I fully experienced it in 5e I found that it can be a bit like playing whack a mole. This can be a good or bad thing, depending on what you're trying to do.

This won't stop players and enemies from dropping quickly, but it will reduce the lethality of combat somewhat and easily allow both sides to continue fighting.

EDIT: One last thing. This is dependent on enemies having the ability to heal themselves in some manner. I know a significant portion of the Monster Manuals do not include any sort of healing for monsters, but consider tweaking as necessary to take advantage of these rules. Not necessarily for every enemy (let zombies be zombies), but instances where it would make sense such as having several kobold priests accompany a dragon or a pack of gnolls that have a few weak healing potions on them. Even 1 hp is enough to be 100% back in the fight.