-Have 1 pose for your running/jumping/etc. and one for your animation montages (will have a slot attached to it)
-Use Layered blend per bone node to make slotted pose play on upper body and non-slotted run/jump/movement pose play on bottom.
-Then plug THAT into a blend poses by bool node that plays the full montage pose if the player isn’t running/jumping/etc.
It will look something like this (Ignore the name “upper body” for the montage pose, it isn’t just for upper body when used with the bool node):
Also, if you want another really easy way to do this, all the Paragons are free on the marketplace and they come pretty much hooked in like this already.