I finally finished the game on easy. I didn't get the good ending, but I was still expecting to get the "Cakewalk" achievement, but nothing was awarded... Super frustrating - the achievements are the only thing that give any sense of making progress in this extremely tough game, and after this experience I think I won't be coming back to it any time soon.

Similarly, all my stats were maxed out, but no achievement was awarded... Do I need to have everything maxed out plus 3 elixirs still with me?

I'm running the current version 1.0.2 on Linux.

Huh, turns out I'm not the first one to run into such problems:
https://steamcommunity.com/app/464260/discussions/0/1697221160919652633/

Issues with the MAXED OUT achievement are known for more than two months, and the devs were planning to look into it back then.

Hopefully the fix also makes it to the Linux version here on GOG...