That is why I take the attitude that skill advance in Traveller has to depend more on what skill you are dealing with, rather than a one-way fits all approach. That is also not the easiest thing to explain how to do in a rule book.
True, the requirements for advancement for each skill level for every skill is totally up to individual perception and can vary from skill to skill and person to person.
For a game mechanic, I believe people do need something simpler than reality and something that at least the majority can live with.
Perhaps the start of a unified, one-way fits all approach version is to stop looking at it from "this is what
I believe" skill such and such represents and looking at what a game mechanics say skill such and such represents.
For example
Level 0 in a skill, any skill, is the competency one has after successfully achieving xx amount of time/experience/instruction.
Level 1 in a skill, any skill, is the level of competency after successfully achieving yy amount of time/experience/instruction.
But I know a Sally who became a skill/level in such and such time! Joe took such and such time going to night school and Jim took half that going to school full time!
Bill got lucky and their dad gave them a job in the company business, Bob had to move up in responsibility after spending much time doing menial work to pay their dues!
Use typical game mechanic modifiers and a task to represent the realism. Some randomness for the lucky and unlucky folk or variations in learning throughout the universe? The random dice results of a task check. Use effect as a variable. DMs based on characteristics that represent people of different abilities. Modifiers based on how you learn (do it yourself trial and error in spare time vs dedicated time in an educational setting). Perhaps even a list of DMs for specific skills that one thinks should be harder or take less time.
Lots of possible variation but it all starts with a unified:
A level zero is someone that has xx and a level 1 is someone that has yy.
If people could set aside their personal notion of what a level 1 is vs a level 2 and agree to let a game mechanic define it, we might have something. Not perfect, but something most people could live with?