Your thriving but hidden civilization needs 3 plausible things:
1. Isolation
2. Resources
3. Cultural pressures
Isolation would most likely be geographic, although a borderland, filled with super evil venomous monsters could do in a pinch. You probably also need an isolation event as well e.g.,
1. Storm blew colony ship off course
2. <PERSON> crashed in remote lush valley
3. Landslide cuts off access route
4. Giant Spiders settle in the boarderlands
5. All of the villages in-between died out in a plauge (our village practiced physical distancing and hand washing, how quaint)
Resources, so Easter island is isolated, but lacks many resources. So you need some place with say lots of easily smeltable metal ore, and fertile land. So they can grow but be self sufficient within their bubble.
Cultural pressure, is probably the most important. You need something which causes the society to overcome, cooperate, specialize, trade (internally), and adapt. 121
Examples of this are:
1. Climate in northern Europe.
2. Needing to create a bureaucracy to manage annual flooding of the Nile in bronze age Egypt
As to where...
It depends what you mean by thriving. There are plenty of pacific islands that are isolated. I would say they not thriving, but small (happy) and stable. They don't have many cultural challenges*. ( Because there is a consistent ready supply of resources (you can just just stick a few sticks of cassava in the ground and 6 months later you have a harvest) There is little need build up an excess and then use that excess to trade and level up.
I think if you look at map of the globe at night, anywhere there are not too many lights will make a good spot. Basically any where isolated. But it would need to be somewhere different than where the target audience lives. Even though New Zealand is the last place in the world (excluding Antarctica) to be settled; it would seem silly to me (I live in NZ) to frame it as where lost city of Atlantis was located. The same goes for Shangri-La if you come from Tibet or northern China.
You know, with a few carefully picked isolation events you could locate your civilization in Switzerland! Interesting and different culture, physically isolated on pretty much 3 sides. Add a plauge and some wolf's in the North, a couple of fires in some library's and council offices burning up records of their neighbors, bake for 100 years or so, and boom hidden civilization right in the middle of Europe!
*I don't want to seem like I am belittling the rich cultures of pacific islands. I just think OP is looking for a civilization with something like metalworking, large temple grounds, that kind of thing. 121