On the other hand, you might have a language with difficult basics, but relatively simple mastery once you have the basics down - I don't know enough languages to give an example, but it seems plausible.

So, yes, you'd likely get different results if you're comparing languages at different proficiency levels, but a rough comparison should still be possible.

My impression, as a native English speaker who has learned German, is that German is much more like this than English. You have to know quite a bit to say fairly simple sentences. But once you know the basics, there are only a couple of advanced concepts to get you to mastery.

[maybe this depends on how you learn. My approach was, roughly, first learn to communicate, then learn to communicate correctly]

To correctly form simple German sentences you need to learn 90% of the non-vocabulary elements of the language. To get to mastery, you have to learn the other 10%.

To correctly form simple English sentences you need to learn 10% of the non-vocabulary elements of the language. To get to mastery, you have to learn the other 90%.

Ranking human language complexity is a much more difficult task than you might imagine.

Still, my native language is Romanian, and I found English verbs easier to learn than French conjugation, even though French is very similar to Romanian in the first place.