Sorry if this or something similar was already suggested by someone else, i was unable to find it.
I think for each skill we need a technical level and a practical level.
Technical level means what the character know about the field, represented by the skill. It’s can be increased by books or by increasing the practical level above the technical level. Technical level should be not rust. Or should rust slowly - but not below the practical level -, this would make skill books forever useful.
Practical level is the character practical knowledge. It’s level can be increased by practicing the skill and if the skill is neglected, the practical level is rusting. Just like the skills works in the game now.
When a skill is practiced, the xp should be doubled (?) until reach the the technical level, because they practicing what they know, they just not perfected it yet.
Skill usage with mandatory skill level check (mechanic, cooking etc) should use the technical skill for checking. If the practical level is lower than the mandatory skill level then the spent time should be multiplied by “mandatory skill level” - “practical skill level” +1. So, you can install that solar panel with technical level 7, but because your practical level is only 3 and the solar panel installing needs level 6, the installing time will be four times of the normal.
Skill usage with failure chance (melee, ranged attack) should be use a random number between practical and technical level. Sometimes you can do that dragon kick, sometimes… well, you should really spend more time in the dojo.