Super fun! Great gameplay loop.
One issue and one nitpick:
1. You can practically softlock yourself if you build too many things (houses, roads) around your starting trees before the game teaches you about the sawmill. Obviously you can always sail to a new island if you ever do it by accident but the game only teaches you how to do that later. My first try I just assumed I was stuck. I like that buildings are permanent, but you might need some safeguarding around that specific scenario in the tutorial.
2. It got annoying having to hold the button for upgrades when I had a bunch of money to spend, especially as you have to release and repress for each subsequent buy. Consider at least making it much quicker to buy upgrades that are very cheap relative to my total money (i.e. allow me to near instantly purchase a 50 gold upgrade if I have 1000 gold)
Other than that, will be keeping an eye out for the full release. Congrats on the great demo.