It sounds like a great idea to me.
So does making it a temple of Tiamat. If you do that? I'd further dress it up as a series of 5 initial temples, each dedicated to one color of chromatic dragon. Each temple having 1/6th of the total loot in it. Including an obvious & lootable magic gem (maybe the eyes or heart of a statue of each particular dragon type?).
The final temple would be to Tiamat herself, include the last of the treasure. and a magic staff - with 5 spaces that obviously are meant to hold gems.....
5 players, 5 gems, Tiamat has 5 heads, hmmm....
On delaying this a few lvs/running the jungle adventure;
I'd do the jungle adventure 1st. Have the portal transport the PCs back in time to the jungle areas equivalent of 1700s Venezuela (IE; a very long time ago based on the worlds current year). Do whatever there. While there? Have them learn of the temple. Maybe it's even an active site! Sadly they don't get a chance to attack/investigate it before they shift back to the modern day.

But they now know where it is! And they know it's full of loot!

Getting there in the modern day, they find that it's been sealed shut.
Not that that should stop PCs. Or give them pause to consider NOT opening it.
However.... Breaking the seal sends an alert to those who don't want this ancient site opened. Namely the metallic dragons! And they send a Bronze/Copper/Silver/Brass/Gold dragon (whichever you think would be the coolest) to
kill whatever comes out of the temple.
Why does the good dragon show up at the end? Travel time.
Why doesn't it go in & deal with the PCs earlier? Can't. Mystically barred from entering the domain of Tiamat.
Why's it fight the PCs to the death? Because it's not interested in negotiating with agents of chromatic evil. If the players protest that they are in fact not evil/not serving Tiamat? It simply doesn't believe them. (the Persuasion roll DC for the party should be about 2pts higher than the max they could possibly achieve

)
Especially not if they reveal their true colors (so to speak) by transforming into young chromatic dragons! Besides, how often do players get to fight & kill
metallic dragons? That's a lot cooler than yet another big blue, etc.
And if they survive, now the PCs are involved in something larger than just themselves. They've allowed Tiamat reentry into the world. They've earned the animosity of good dragons. And they're enjoying the blessings of Tiamat.
Edit: Ooh, an afterthought! During that week they're turned into dragons? You should have them go attack a little town called Greenest!