You should be able to achieve this using blueprints, by using the Play Montage node. Promote the Starting Position pin to a variable, set the variable to your desired value after the On Completed pin and trigger the same node again. Just make sure not to trigger it every tick. When there’s an interruption, you can simply cut the montage off by using the Montage Stop node, and also reset our variable to 0 while you’re doing that.
You also don’t need montages for this and can just use normal animations as well, again, with blueprints.
However, since this is gonna be an idle animation, I think it’d be better for it to play in your animation blueprint. So I would recommend just playing that looping part of the idle animation after the first part by having the first part in a state that’s connected to the entry of your locomotion, and connecting it to the looping part that’s gonna be in a separate state with a one way transition which’s condition is set as automatic rule based transition. And make sure that the second animation is set to loop from the animation asset.