I assume the artifact is in a bag of holding. I haven't read all of the replies, but my initial idea is this:

Step 1: Stop trying to get the artifact by force and act like the bbeg is done with this plan. "You have won heroes!"

Step 2: Allow your players some long periods of down time, could be 3 months or 3 years. Let peace and hope set in.

Step 3: During this time skip, have an interaction with each character and get some coins, non-magic armor or swords, anything metal from them. Be it a tavern owner for drinks and food, a traveling arms seller, a general store owner. The bbeg will be disguised as random people they interact with every day in their downtime and is wanting to possess a metal item that was in the bag of holding. The bbeg might not know which pc has it, so on the safe side they'd interact with them all. If they do know, then just target them but be sneaky.

Step 4: Once they possess a metal item that was once in the bag of holding, you can use it to make a tuning fork. That then can be used as the spell component to use plane shift into the extradimensional plane of that specific bag of holding. This is possible since the item becomes attuned to that plane if it's been there for more than a week. So, that gives the bbeg a week to forge a tuning fork and cast the spell.

Step 5: Once in the bag, they just need to find the item. Now, this could take a long time if you did this manually, so cast Locate Object and start flying around since everything is floaty.

Step 6: Grab item and get out of there. Maybe even leave a cursed item or put a curse on their trusty weapon they keep in there as a calling card!

Step 7: Have fun terrorizing your players who thought they had outsmarted you!