Thanks!! Yes, I am a native English speaker.

Oh I never thought about resource richness! Thanks.

Oh wow - thanks! The perfective is something I didn't study before I went, and I learned it from primarily hearing it in the negative (e.g. 宿題をまだやっていない？！ - mom to her kids). So, I was kind of familiar with the perfective but this crystalized it for me. I didn't know the 言っている nuances either.

That video is so helpful - thanks for linking!!

Thanks!

Thanks!! I like that analogy.

Thanks!

I like this article's suggestion: https://japaneselevelup.com/the-best-response-to-nihongo-wa-jouzu-desu-ne/

tl;dr: 日本が好きですから！

Personally, I usually deny it:
いや、忘れてます！
いや、まだまだです！

And then say something ironically difficult:
片言だけですよ！
最近使っていないからレベルが衰えています。
Because it's entertaining.

Edit: formatting

Thanks!