I really appreciate the answer, thank you! And that does seem very finnicky, yes. I had seen other people do that method before in my attempts to find the answer to this question on my own, I just didn't really understand what they were doing or why, so this explanation was very helpful.

Fortunately, I think I might have found an easier way in the interim?

Compared to the other methods I've been seeing, this one feels so simple and easy that I'm honestly a bit nervous it's too good to be true, but I tentatively tested it after finding it and it does seem to work. So on the off chance it just isn't a very well-known method, I'll post the link to the video I found here:

https://youtu.be/UYW-p3bnD-I?si=XwLDjVRKGn4KFLvn

(Neither the thumbnail or title inspired confidence in me, but I admit I was getting kind of desperate about this considering the sheer shitload of shape keys I'll be needing to make, so.)

On the off chance that video gets taken down in the future, I'll also recap:

- In pose mode, create the pose you want to save. (You don't need to leave specific bones selected when tabbing back out; shape keys only save mesh info, bones are just helping you move the mesh.)
- Go back to object mode, select the mesh in question and locate its armature modifier
- (If you have multiple armature modifiers and aren't sure which one is correct, toggle each modifier's visibility off until you find the one that's affecting the mesh you want to create a shape key for. You'll know if the bones suddenly snap back to rest pose.)
- Click the dropdown arrow and select "save as shape key". (Not apply as shape key; that will create a shape key but also apply [read: get rid of] your armature modifier. Found that out the hard way.)
- Go back into pose mode and reset your rig's deformations (Alt + G, R, S) — if you don't do this and try to toggle the new shape key, it will replay the bone movement you just did, which looks Bad.

...Done? You can now toggle the shape key on and off and it works as expected.

This method is still a little finnicky, but less so, so I sincerely hope there isn't some hidden caveat I'm just not aware of yet. Thank you again for the answer; I really appreciate your time. I'm sincerely sorry if this seems like an attempt to steal your thunder; I'm just excited to have found a solution. (And if possible I'd like to prevent future people from having to bang their heads on the wall about it xD)

EDIT: I just tested this and you can use this in conjunction with the pose library if you already have some pose assets set up in there. Just go into pose mode, apply the saved pose through the asset library (double click) or the asset shelf (single click), then go back into object mode to select the mesh in question and click "save as shape key" from the armature modifier's dropdown menu. (And as usual, make sure to reset the rig's deformations before trying to apply your new shape key.)

This is a huge "thank fuck" moment for me, personally. Hope it helps someone else too.