Make mobs drop all their stuff on death

There is a simple way to do this. The chance of a mob dropping its armor and held items is determined by an NBT tag. This means you can make a procedure that triggers when an entity spawns, checks if the entity is a monster, and, if it is, overrides that NBT tag to be 1. (Guaranteed 100% chance- by default, it's more like 0.02, or 2%.) You can do this by either using a data merge command, or by using NBT modifier functions in MCcreator, and it'll work for any monster entity that can have stuff equipped. (If you look at the items tab of this command generator, it should give you the tags you need.)

You just need a procedure with the 'when entity spawns' global trigger. 

Then you need an 'if' bracket that checks if your entity has your custom tag.

If it does, inside the if bracket, you want to run a command that changes the Hand-drop-chance NBT tag to 1.0, using a command that targets the nearest entity. (You would use /data, which allows you to override NBT tags.) The specific command I use for this is:

/data merge entity @e[type=wither_skeleton, sort=nearest, limit=1] {HandDropChances:[0.2f],ArmorDropChances:[0.2f,0.1f,0.1f,0.2f]}

...Where you replace type with the registry name of the entity you want. 0.2f gives a 20% chance of tropping, and 0.1f gives a 10% chance. You would want to replace all of these with 1.0f.