In sandbox mode a separate xp multiplier should be added for each skill, as opposed to an overall multiplier, as some skills are easier to train than others. Either that or the devs should change the experience you get for certain activities, so that an average survivor can hope to max out a skill after working at it for a few months. Why do we want a change? One example: I have a character that has survived for over 3 months and I have been foraging almost every day; my foraging skill is level 1...
Here are the exact values, by which I have increased my experience multipliers i the game (by going into lua files), in order to make it more enjoyable, but not op:
Fishing----------------------------x9 Trapping--------------------------no change, I don't use trapping Foraging--------------------------x10
Fitness----------------------------x180 for sprinting, x100 for swinging a weapon Strength--------------------------x90 for heavy load, x50 for chopping trees
With this set up in about half a year a dedicated player can hope to max out most of the important skills in the game.