I read about treasure islands many times and yesterday visited one of the free islands by Pange Plays? It was very interesting, looked like it had everything a player would need to get an island to level 5. I think it had 6 free islands with all the raw materials, bells, gold nuggets, star fragments, diys etc . Some things looked like it was better to avoid, such as the bridge construction diy and I avoided the section with creatures. In addition to bells, it had Royal Crowns - 20 will get you 6 million bells at Nooks Cranny. The web page constantly listed who was on each island, along with the current dodo code and a map showing where things were located. The only down side was waiting often as players come and go , and we did get booted when someone left silently. We ended up spending over an hour, looking at various things available on several of the free islands. We live in USA and early morning seemed to be a better time, all 6 free as islands have status updates so you know how busy they are when you visit the website. The free islands are impressive, but there is a paid subscription, about $5 a month for artwork and other harder to get items.
I often read about problems after using treasure islands, but the DIY seemed OK, as well as the items we picked up. Overall not much different than visiting any other island with a giveaway. Well, the island looked different, flat with no sea.
Here is the website, but be careful, you visit at your own risk. I can not say it is safe, it seemed OK to me.
Update: We spent several hours learning about Treasure Islands, using Twitch may be better but far too many hoops to jump through for me. The Pange Plays website is easier. The biggest problem for me was it made the game too easy, it may be OK if all I wanted to do is decorate an island to impress others. Maybe if you needed something like fish bait, but then they have fish you can donate, why fish if you can just grab what you need. The free islands I listed do not have art work or items you purchase but everything else. It was interesting, just need to be patient, other players are always coming or leaving. We are restarting my island again, my favorite time in the game is the first day, not having anything but lots of ideas and challenges. It is nice to know about Treasure Islands, and I do not see it any different than people visiting other islands to get free stuff.