Hi, I really think the content from this mod should be in the base game, as it is just tweaking the values of XP in the custom sandbox. Sadly, this mod doesn't seem to work 100% but I believe the dev team would be able to implement this.
DISCLAIMER: I did not create this mod. I just think the features it adds are essential for the sandbox customisation experience (as a flat multiplier covering all skills isn't sufficient). Seems that the mod author has gone dark thus my decision to post it here.
https://steamcommunity.com/sharedfiles/filedetails/?id=2827283808
In case the mod gets removed, here is the description of the mod.
Basic Features
Ultimate XP Tweaker allows you to set an XP modifier for every individual skill in the game! For example, if you think most skills level up fast enough, but Fitness and Nimble are too slow, you can make only those skills level faster.
The multipliers used by the mod are properly applied in every case, respecting other built-in multipliers in the game, such as the global XP Multiplier, "XP Boost", skill books, Fast Learner, Slow Learner and Pacifist.
Advanced Features
Power users may also use the "Advanced" sandbox settings tab, where more intricate settings are located. These settings can safely be ignored by users just looking to multiply the XP for a certain skill. The following features are listed under this tab:
Modify XP Boost: +X%
Every XP Boost value in the game (normally called +75%, +100%, +125%, etc) may be modified, per individual skill. For example, a character starting with 0 Carpentry normally gets only 0.25 times the amount of Carpentry XP that a character starting with 1 Carpentry gets. If you think this is too extreme, you may change this to any value of choice. A good starting point could be setting the boost for 0 starting points to 0.6666.
Level 5+ Aiming and Reloading XP
Characters with Aiming or Reloading levels of 5 or higher gain a large debuff to their XP gains for those skills. Optionally, this mod allows you to change those.
XP Derandomizing
In vanilla Zomboid, XP gained from moving has a random chance to apply every frame. This mod contains an option to derandomize these and gain XP at a regular rate instead. This includes: Strength XP from walking while at >50% carrying capacity, Fitness and Sprinting XP from running, and Nimble XP from walking in combat stance. This is enabled by default, with the XP gain rate being exactly the same as vanilla Zomboid would be on average, but all of this can be configured. As an exception, Lightfooted/Sneaking XP or Fitness XP from walking at excessive exertion or exhaustion are not derandomized, because those mechanics are part of the game's Java code and not the moddable Lua code.
Reducing XP gains
While the configuration of this mod only allows you to increase XP multipliers, and not reduce them, if you really want to decrease XP for specific skills, this is still possible by the following workaround. Set the vanilla "XP Multiplier" under the "Character" tab in the Sandbox Options to a value lower than 1.0. Then adjust the mod's values accordingly. For example, if you only want Cooking to level half as quickly, set the vanilla XP multiplier to 0.5, and set all the skill multiplier values from this mod, except for cooking and strength/fitness (the vanilla multiplier doesn't apply to passive skills), to 2.0.
