Leveled NPCs availability week to week

In my weekly Stonehell/Rappan Athuk game, there are a number of leveled NPCs that regularly join in on our adventures... but not always. Given that each of these NPCs is an independent character, and not a paid hireling, I wanted there to be situations where they wouldn't be available, for whatever reason. To keep it simple, I decided a simple morale check would work, with anything 5 or under would mean that NPC wouldn't be available this week.

Thinking about it more, I wanted to add a little something to the check.

Charisma modifiers were a good place to start, especially as my character Nick has a penalty. It just made sense.

But then I thought about other things that would impact whether someone would want to brave the dangerous dungeons with these particular murderhobos. This is what I've come up with.

Make a morale check. A modified 5 or under means the NPC is unavailable to adventure. Under a 2 means it is very unlikely they will ever rejoin the party, and may organize a rival group)
-1 Reduced to 0hp last adventure
-1 Someone died
-1 Unsuccessful delve (no magic items recovered, not enough treasure to cover expenses
+1 Successful adventure (a magic item was recovered, profit was earned, quest complete)
+1 Leveled Up (NPC gained enough XP to level up)
+3 Very successful adventure (NPC got a magic item)
+/- Charisma Mod of group leader (not necessarily the highest level or most charismatic)
+X Bribe (varies by what's offered)