Recommended third party tools for generic rig IK?

I want my generic rigs animator to have two layers, the basic layer that contains all the premade animations I made, and an IK layer. Pressing a button simply moves the weight of the IK layer to 1 or 0. This isn’t exactly something that is easily done in with Unity’s Mecanim that mainly supports humanoid rigs. So are there any third party tools for this? I’ve been eyeing final IK to see if it’s something that will work for me.

Are you referring to the fact that you are not using a humanoid type of model?
Try Animation Rigging

Technically I’m using a humanoid model, but the model uses 5 separate rigs that control different limbs of the body. I did this because I intend to be able to swap out limbs with other kinds of limbs. So each rig is just a limb (one for an arm, one for the torso, etc). As I’m typing this I realize I could probably just create separate blender projects to just import a limb as it’s own individual asset (or import it in a way so that it acts like that), set it to humanoid, and just set each bone within the humanoid rigging system.

But if you want to import it as a humanoid, you need a complete humanoid skeleton. Suppose you have a complete skeleton for each of your five limbs (even though you only animate and display parts of them), this might be a bit of extra overhead.
I’m not sure if your different types of limbs have different skeletal structures, otherwise, perhaps you could use a non-displayed character to play the animation and then attach the limbs to it?

Yeah you’re right, trying to implement it right now and I do in fact need an entire humanoid skeleton. And yes the legs will have 4 types of skeleton structures and the arms will have two, although the only thing I plan to use IK on are the arms which will usually just point at a target from a distance.

OK so to follow up FinalIK did in fact fix my issue. Instead of animation layers there is just a weight value on the component that poses the bones to the target or to the current animation state. Very convenient.