dnd 5th – How can I combine the number of low CR creatures so that their use does not remove the fight?

I have a whole series of encounters built around different types of spiders, controlled by big bad guys serving Lolth.

At the bottom of this hierarchy of arachnids is a large number of chitin. Thematically, they fit the scenario, but I was not happy with my first session. Many of them were first killed with area effect attacks, which was great, but then I tried to get them moving and attacking in groups, and I I have mastered mechanics and mathematics. Everything went well, but I would rather have a little more structure than aileril.

What I might like is a way to take statistics of the number x of creatures (chitin in this particular case) and combine them into a group, so that the stats of creatures x are reflected in the group.

I've read the section on mob combat in the DMG, and I think that's the kind of direction to take, to combine the number of creatures in a group, but the DMG calculations seem complex to manage on the fly, and I do not do it. I do not want to have to consult a table. It would be nice to have a formula or method to translate x creatures from a group into y to inflict damage, and so on.

My question is: How can I combine the number of low CR creatures so that their use does not take away the fight?

Plus, I've read this question and I do not think it's a copy of mine, because I'm talking specifically about low-CR creatures.