I estimate the threshold distance is in the ballpark of 10 metres/30 feet (see screenshots at the end of this post).
Well, not all of it is complicated, so let’s get the “simple” stuff out of the way first: 2.1 Regular crit bonuses +50% critical damage after using Rakk Attack is a whole different story which is explained in the FL4K section.
After accessing the Obelisk's Inventory and selecting an Artifact from the player's Inventory, there will be an option to “Tribute”. After recovering an Artifact from a Cave, it can be brought to an Obelisk.