Thanks and good to be back!
I think most of what you say here is the most common situation. Most players
that have played awhile tend to get at least ok shoulder turn and pull to the
ball, which will take care of hitting thru the ball due to the forces involved.
So most need to be better at the up and across aspect of the swing.
Folks who are focused on the "thru the ball" concept don't see these players as
needing work and mostly just see them as needing more practice to keep the
balls in (or tighter strings etc..
And yes, there are those out there who don't get much or any shoulder turn with
good pull to the ball. Often these same players will also tend to pull to the side,
leading with their head and "pull off the ball" as it is called. So yes that happens
with some self taught modern players.
But with our instruction...not so much.
Good modern stroke production highlights pulling the the ball and adjusting the
amount of shoulder turn largely based on how much power you intend. So you
get very good shoulder turn and this leads to forces that get the racket head
strongly thru the ball as it travels up and across. You don't have to teach or
focus on the "thru aspect", as it is created by a solid pull to the ball.
That where your comments throw me a bit. You say you slow the swing speed
and go for more spin. Imo that is not the best way, and I tend to think of
it as even increasing the swing speed (slightly) when going for more spin,
along with increasing the up aspect of the swing (not when on the rise though).