The purpose of Mass Combat Rules is to treat a large number of units as a single unit so they are manageable.
Attacking
I was thinking of using Carnage Amongst the Stars (unless Carnage Amongst the Tribes was released) with Open Legend’s attack roll. The number the PC rolls equals the number of Foes the PC killed. Exploding Dice could be really fun!
Being attacked
I’m at a loss on how to handle the PCs taking damage. I’m thinking of using a Time element, the longer the PCs take to handle the foes, the more the foes do something the PCs don’t want to happen.
Invoking Banes
Many of the Banes make the foe worse at something or the PCs better at something vs the foe. I have no idea what do to with these.
I don’t want the PCs to easily end the fight early by invoke Fear on the group, but this would be a cool way to end the fight after 2/3rd of the foes are dead.
Situation
The PCs will be defending a smallish sized village with about 500 NPCs. I would like to have about 100+ Foes attacking the village, mostly to pillage, murder, and kidnap NPCs. The village will most likely be on fire in a few places.
Without having a closer look at your inspiration, it’s hard to tell for me how this could play out, because your description and explanations are a bit bare-bones, but as a general advice I would refrain from players playing as their individual characters in Mass Combat. A) that makes it hardly mass combat and B) it makes it very hard to simulate, which is why most games resort to army vs. army for mass combat.
If you want to create large masses of enemies and want to treat as a single enemy/entity, than I wouldn’t call that “Mass Combat”, but rather Swarms or Troops, so I’m not exactly sure what you are aiming for here.
I like the idea of large scale combats and have been trying to figure something out myself. My concept is to have the battle mostly be treating armies a one entity and having them make attack rolls against each other. In between these sections, you would have the PCs doing things on a smaller scale, like assisting a general, creating a new strategy, or caring for the wounded. The smaller scale actions would have impact on the larger scale roles, like adding advantage or disadvantage.
I might add Horde and Mob as other viable name options for groups of small and larger creatures.
I’m working on setting up a campaign so I can’t say I have. Depending on how detailed you are looking to get, you could try converting or adjusting the following D&D 5e UA rules as well as the associated DMG, p250.
It includes things such as morale, commanders, and PCs within a unit.
PCs could be their own unit or commander of a unit.
You might consider adopting a horde/mob/troop/swarm of minions as well.
In terms of what you’re aiming for, isHP = number of creatures in unit about the gist?
Personally I think that value would be too low. You might try HP = number of units * lowest HP value of individual or use the HP of a minion version. The unit could gain adv = to the unit’s commander’s Presence attribute and for every boon or bane affecting it you could give dis/adv±1. That’d be an easy solution for morale and boons/banes. (Perhaps healing would be an exception?) Certain feats could increase the amount of dis/adv (e.g. multi specialist feats).