Since you are not in the middle of combat or anything I'm not going to have you make a lot of rolls to break through the wall, it will take some time and quite a few swings but you are able to eventually break through the 6-inch thick stone wall.

After the hard work Pepin takes a moment to catch his brearth before moving through the hole into the treasure chamber.

Each footstep makes a little noise and slides slightly as the coins chink and shift under your feet. It is considered difficult terrain.

With a treasure hoard this big, Pepin wonders if maybe other parts of the Orrory of the Wanderer could be here. He starts to look around for any of them.

Investigation: 13+4 (Guidance)+flash of genius= 22

Yes indeed, after a few moments of searching you find another part of the Orrory of the Wanderer.

Randall:

Likewise beginning to search, Randall looks for items of use and/or power, portable monetary treasure (gems, etc), and clues as to the origin of this treasure.

Perception:  18

Post which gems (type, value etc.) you are searching for and you think some of this treasure may of belonged to the Black Dragon/Dracolich you defeated. All treasure hordes grow over time but this one looks like a treasure horde was added to a treasure horde, and then another, and another etc.

Randall:

As many powerful healing spells require gems of great purity  and value, Randall searches for diamonds.  For items of power, he would be interested in anything dedicated to these holy gods—specifically Lathander—and magical plate and shield would be nice.

Pepin is extremely happy with finding the dimensional loop; he only needs to find two more components and the Orrory itself now to complete it.

Zenodor follows the others into the treasury and looks for any interesting magical items.

He then cast Demiplane to create a small treasury room the group can access from here, and later from the city. A door magically appears on a nearby wall.

While he does so, he says to the others : "Let us gather up what we can. Any wealth and items which we cannot easily carried can be stored in there. We can then later access this demiplane from the city. I can keep this open for an hour at most."

Once his search for magical items is over, he will just carry / throw / shovel gold and gems into the Demiplane treasury 

Randall finds several nice diamonds and gemstones, while looking a yellow gemstone catches his eye, it is half buried under some coins but when you brush them aside you see the gem is embedded in the chest of a polished suit of armor.

Pepin notices a finely crafted book near where he found a part of the Orrory.

Zenodor is excited to start shoveling as much of the treasure as he can into the demiplane but that wavers a little as he glances around the 80 foot by 80 foot room packed with mounds of coins and other valuable objects and at the 30 foot by 30 foot demiplane.

Randall:

Digging up the armor, Randall examines it closely, hoping to perhaps find a matching shield.  Regardless, he spends some time with the armor, looking especially at any religious iconography.

He puts the diamonds with his other spell components, confident his powerful spells will have the components needed.

Pepin quickly picks up the book to see what it is about, and why it is so beautifully crafted.

After which he'll help Zenodor to move treasure to the demiplane.

Quote from Brian_Avery >>

Zenodor is excited to start shoveling as much of the treasure as he can into the demiplane but that wavers a little as he glances around the 80 foot by 80 foot room packed with mounds of coins and other valuable objects and at the 30 foot by 30 foot demiplane.

 (Theoretically, as long as the treasury isn't stacked nay higher than 4ft, we can fit it all into the volume. But even if we only get a tiny fraction, it would still be a significant amount of wealth to take back)

(Does Z find any magic items?)

After Randall examines the armor with the yellowish gem on the chest you notice it has the 'shield' rune etched into it.

Pepin starts leafing through the book and has a hard time putting it down as it contains some interesting ideas, puzzles, and thought experiments that challenges and engages his mind.

Zenodor shovels lots of coins and treasure into the demiplane but has yet to see the floor beneath his feet, it's still coins and treasures and such. After a while he uncovers a length of char blackened wood which seems out of place in this treasure room and when he picks it up to examine it further he realizes it is a rod with a clean, white skull affixed to one end.

Pepin is extremely intrigued and puts it in his back to study later.

Randall:

Unfurling the flying carpet so he has a good level place to set the treasure and help Zenodor move it, Randall starts directing it to help his friend move the treasure as he desires.

Once that is underway, Randall begins changing out of his armor and putting on the new armor with the shield rune.

Tempe, searches the area, for something that peaks his interest. "If you will, reserve some, for a donation to my temple." 

While moving coins into the demiplane Tempe uncovers a finely crafted staff. At first he thinks of putting it aside as it isn't coins or anything of obvious value, but then he second guesses doing even that. The longer he holds it in his hand the more he likes the balance and... just overall 'feel' of it.