"On the other hand, you might have a language with difficult basics, but relatively simple mastery once you have the basics down"

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.

I'm not convinced by this. If you're just aiming to communicate in German, you can ignore noun gender, valency, many tenses, etc. It's all stuff you'll need to come back and learn later.

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

That's not the point. The point is that in order to communicate simple things correctly in German, you need to learn a lot more than to communicate simple things correctly in English. But to have correct mastery of German, it's not a large jump from there.

It seems like you're saying, once you've gone through 90% of the effort to learn a language, you've only got 10% left (as opposed to 10% for basics then 90% to master).

Yes, that's right, except for the important part. I'm sure the percentages are off, but here's the basic idea:

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%.