I bought the game, downloaded the donation build, but is still the same as preview build. That means I spen 5$ for something that was free?
No you get future builds too, which from now on is out of "preview".
Current build is v0.0.5 which is a mess, but there should be a stable release here soon.
Sorry it's been slow, but the last public release on new years was the first public release 2 years in some cases, months in others.