Unfortunately, in Game Maker, at least currently, there is no visual UI editor. This means that I have to manually place each button at specific coordinates. Then I need to compile the game, see how it looks, and if something is wrong, repeat the process. And so for each available resolution.

At some point, I started using a special extension that allowed me not to recompile the game every time. This slightly sped up the process, but still didn't completely solve the problem and didn't save much time.

The save system in a strategy game with hundreds of variables is a nontrivial task.
I'm proud that I managed to implement exactly what I wanted. The game only has one save slot, but technology and characters are carried over between chapters. Of course, players can replay chapters as they wish.

Generally, a strategy game is essentially a collection of arrays and loops; lists. Therefore, I didn't reinvent the wheel, I simply save the objects at the current moment. However, then, when the level is recreated on reload, I simply delete everything and load the objects and their variables that I saved. It's crude. But it works.