Copy the video below.

Then add the nodes shown in the image below to the FindHarvestAnim function you created while following the video above.

Next create a new float variable called LastCollectionTime then copy the image below.

Next add the nodes shown in the images below.

You can now go to any unit blueprint and in the Class Defaults add entires to the HarvestAnimations setting, there you can set the resources class, then set the animation montage you want the unit to use when harvesting that resource.

You can set multiple animation montages for the same resource and it will randomly select one each time the unit harvests that resource. You can also set seperate animation montages for each resource type.