Alternatives to blend mask

Hello! I was wondering if there’s an alternative to manually setting blend profiles when it comes to animation masking.

The main issue I’m facing is that my animation blueprint is quickly bloating due to how many slots I need to use in order to make the sequences work how they’re supposed to. I have a few animations that are full body type sequences but do not have any key frames in their right arm in order to keep the weapon held correctly; sometimes it’s the left arm, right forearm, left forearm, just the right hand, etc…

My initial idea was to just not export these limbs when exporting the fbx, but unreal seems to default the bones to the skeleton pose instead of what’s playing before the slot node.

In this example here I setup an idle loop with an animation playing on DefaultSlot that does not have the right arm exported, but this sadly is not enough, even if I tick “always update source pose” in the slot node.

Any ideas?
Also, I don’t want to sound crazy, but I swear Unreal used to work like this?