To begin with I feel like the Devs should find a way to keep players from doing this or add a check to see if the file has been edited, because it is a very easy way to unlock all the achievements.

Continue at your own risk to your save file.

Go to file explorer and follow this path

C:\Users\[Your User]\AppData\LocalLow\MisterMorrisGames\Rusty's Retirement

In there you will find some files
You are looking for one that says something similar to "H3-2024-5-4-16-56-40.txt" (This is mine your's will look different)

MAKE A BACKUP BEFORE YOU CHANGE ANYTHING IN THE FILE

Open it with notepad or Notepad++ (either will work fine)

Simply ctrl + f to look for what you wish to edit

In this example I want to give myself more fossils, so I change the value from 0 to 9999

This is what it looks like to begin with
"fossils" : {
"__type" : "int",
"value" : 0
}

This is what it looks like after editing
"fossils" : {
"__type" : "int",
"value" : 9999
}

You can also unlock anything by changing it value from false to true.

In this example I am unlocking a carrot

This is what it looks like to begin with
"cropCarrotUnlocked" : {
"__type" : "bool",
"value" : false
},

This is what it looks like after editing
"cropCarrotUnlocked" : {
"__type" : "bool",
"value" : true
},