This mod allows you to do another run for achievements while not affecting your actual list of achievements on Steam.

The game will think that you didn't receive any achievements yet.
Your achievements and stats will be stored in mods/saves/CustomAchiHandler.txt
Simply remove that file to reset achievements again.

The mod also makes the game think that you didn't complete any heists yet.
When you receive an achievement that you don't have on Steam, you'll get it on Steam as well.
All of the heist completion counters will also be increased as you complete heists.

When you first start the game with this mod, you'll instantly unlock achievements for levels and infamy.

Once you remove the mod, everything goes back to normal.

Install to 'mods'.

The mod does not mess with your save file anyhow, but you can make backup copies if you want to feel safe.