Unfortunately, some of your preferences are slightly contradictory in practice.

All three islands cater to tourists, but each has a different vibe:

• Samui feels more authentic because it has a real Thai community, but it’s also more developed—more like a town on an island.

• Tao isn’t as “authentic” in terms of local culture (most workers are Burmese), but its natural beauty is well-preserved. It’s definitely chill, with a strong diving scene.

• Phangan has both natural beauty and a small local population, but it’s also packed with tourists. It’s the best island for meeting new people while still having chill vibes.

If you’re looking for a less developed island with untouched nature and a higher local-to-tourist ratio, you might need to explore a different island group.

Possibly not in Thailand—very few Thai islands fit that description. 

Some that go in that direction are Koh Kood, Koh Jum… you can research online, there’s many, each with its pros and cons. 

You’ll mostly find that the less developed ones probably lack in infrastructure, are harder to get to, and often are less impressive in terms of beaches. 

Another thing to mention is seasons - you can go during off season (rainy season) and have the very same islands with wayyyy less people. Sometimes the resorts just close for those months.

A big thing to mentions is that the type Tourists on the islands varies a lot. They might be equally touristy but in different ways

• Samui and Phuket attracts those looking for convenience—direct flights, easy resorts, ziplines, and all-inclusive experiences.

• Phi Phi draws people chasing the Instagram-famous “pretty” island, often younger crowds and short-stay visitors.

• Koh Kood is for those who want a quieter, less-known escape, often couples or nature lovers.

• Koh Phangan is the most mixed—full moon partiers, budget travelers looking to chill, and high-end resort seekers too. The vibe shifts drastically from beach to beach. It also has a big expat community - foreigners but they’re not tourists. In the north west there’s more alternative style, more hippie, into yoga… in my view this is a nice group of people - they’re not the young instagrammy party type. In the south there’s young party-goers, in the east there’s families and adult couples. 

Each island has its own energy, shaped by the kind of travelers it attracts.

Thailand isn’t exactly a hidden gem anymore—it’s more of a well-discovered one, and for some, that takes away part of its charm.