iota VS SNAP

The good news is that there's a much easier solution: Run a fixed amount of steps per second, regardless of framerate, aka, Fixed Timestep, for that I recommend using iota, it's on github and open-source. Then you can easily decrease the amount of Steps per second, using the set_time_dilation function, iota also has built-in interpolation functions, so even if the amount of steps decreases to a fraction of 60/s, you'll still get a smooth image.

That parsee is out of date, use this https://github.com/JujuAdams/SNAP/blob/master/scripts/SnapBufferReadCSV/SnapBufferReadCSV.gml

There's no convenient in-built way for GM to perform a "deep copy" on a set of arrays/structs (which is bonkers to me but here we are) so I made a script that'll do the job for you: https://github.com/JujuAdams/SNAP/blob/master/scripts/snap_deep_copy/snap_deep_copy.gml

Finally, does my JSON encoder work for your savedata? If so, it'd be useful to add this as debug output for your users so that you can see what the savedata is when the system fails (though no guarantees that my code doesn't also have issues - please use try...catch and save the crash data in the event of an exception)

Use structs to save and contain all the game data. Then when you save and load convert that struct into a json using this https://github.com/JujuAdams/SNAP

You can look into using the library, SNAP to import the JSON into your game. It's super easy to use.