Ok so a very simple skill system, or VSS:
For any given 'skill' or broad collection of conceptually related tasks you are either non-proficient (novice), proficient (apprentice), expert (adept) or masterful (master).
The DM will ask you to roll an ability check to see if you succeed at a task that is simple, average, difficult or impossible.
If your level of proficiency matches the difficulty of the task, you roll 1d20 and add your ability modifier.
If you are two or more steps below the required proficiency, you cannot attempt such a task.
If you are two or more steps above the required proficiency, you automatically succeed at such a task.
You have advantage, and roll 2d20, keeping the best, if you are one step above the difficulty.
You have disadvantage and roll 2d20, keeping the worst, if you are one step below the difficulty.
The DM never has to pick a DC: this is fixed at, say, 10.
So, an example 'skill' might be blacksmithing. A simple task might be fashioning a nail, an average task repairing a weapon, a difficult task could be fashioning a masterwork weapon, an impossible task would be making dragonscale armour. Novices can make nails and attempt to repair weapons, but they won't be making dragonscale. Your average village blacksmith could only make you fine armour, not dragonscale, and nails are easy. A reknowned blacksmith can attempt that, and can churn out nails by the dozen without breaking a sweat.
Pro: Simple, bounded, customisable and ability-independent
Con: Not sure on the math with different ability modifiers, not sure if auto-success or fail are to everyone's taste
That isn't what I'd consider "simple." It seems like you're creating a very complicated system to avoid ever having to pick a DC. I'd rather have just two categories, one for tasks that can be done untrained (trained people don't have to roll) and one for tasks that require training (untrained people can't even try). Then assign DCs within those categories.