Well ... basically, hearing music is hearing sound waves, moving air. Our ears and the "interpreting apparatus", the brain, are in no way linear. What we call loudness depends on the relationship between average level, peaks, dynamics and frequencies. Furthermore, most musical pieces have a kind of "meta rhythm" in the form of musical bows (arcs) done by phrasing and loudness changes. And there are psychoacoustic phenomena, for example the fact that a tone sounds louder if it follows a part with lower volume level.
If you produce very similar musical pieces, it is indeed possible to set even loudness according to measurements. You would keep your RMS level, peak level and dynamics always within the same ranges. Without big differences in the freqency ranges, all pieces could sound equally loud and living. You may be able to use "automatic" limiting, where the max peaks are given and everything else follows. This was often tried in the past as a scientific approach for automatic mixing and mastering.
On the other hand, mastering engineers are payed for their skills to bring out dynamics and control the levels as well as frequencies to make music interesting and compatible to all systems which will play this music. These people are supposed to have golden ears, an acoustically perfect listening environment, musical feeling, and experience. There would be no mastering engineers in the world anymore, if a plugin or a technical method could successfully replace them.
So, what do you want to become, a scientist or a mastering engineer?
(That's a silly question, I know)
The easier way:
Put your pieces into one project in the order you want them. Let them play, listen, adjust what you have to. Use your ears, switch your brain to "overall" mode. If your loudspeakers and your room are good and in good balance, you can come close to a perfect result after some training. Closer than you may come by measurements alone.
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