this leads me to a very simple question: on a standard
day, without any winds, what would be the optimal
altitude for [cruise] speed in a normally aspirated airplane?

That's not really a simple question. You haven't really defined
optimal.

If by optimal you mean highest available cruise speed, then it's sea
level. That's because most normally aspirated engines may be run at
full power continuously.

If by optimal you mean highest available cruise speed at maximum
RECOMMENDED (not allowed) continuous power, it's the highest altitude
at which the engine can develop maximum recommended continuous power.
Depending on the prop, that might be anywhere from 6000 to 8000 ft
(density altitude of course) for 75% power.

If by optimal you mean best fuel efficiency, it's the altitude where
maximum available power produces an indicated airspeed equal to the
best glide airspeed (unless the engine/prop happens to be inefficient
in that condition) - in other words, this will vary with the airplane,
the engine, and the operating weight.

Michael