Opening move choices for long-term learning

Here is my attempt to to ask this question in a meaningful manner so as to elicit meaningful answers …

What is the narrowest set of early opening move choices for the developing chess player that follows general opening principles and produces the variety of instructive pawn structures that are recommended for long-term improvement, and does so in the shortest number of moves?

Generally repertoires that begin 1.e4 e5 or 1.d4 d5 and are not systems.

Then you might ask whether you should pick the Spanish vs Italian, or pick the Slav vs 2...e6. Either are fine (lead to many fundamental structures and follow principles).

"Shortest number of moves" is a confusing criteria that doesn't really fit with what you're asking, so I ignored that.

 

Thanks,

I am looking for the simplest comprehensive repertoire to learn and apply general opening principles and getting exposure to a variety of instructive pawn structures without being drowned in theory and less instructive earlier deviations. I know it is impossible to satisfy all of these goals, but is there a “20-80” solution? (A repertoire that offers exposure to maybe 80% of the most instructive pawn structures, but with only 20% of the theory).

I like the classical chess choices, but there is still a massive variety of choices after move 3. For those of us with little time, what is the narrowest path to achieve the oft-stated beginner goals of following general opening principles and exposure to a variety of pawn structures, but want a reliable, lifelong, but somewhat simpler repertoire that is still instructive for long-term improvement?