I'm always under the impression you've this kind of insight from a native speaker.
Yeah, I guess you're right about that. My Hokkien is very limited in vocabulary, but for the vocabulary I DO know, it's fluent, and my knowledge of nuances in meaning between two words which I do happen to know is indeed at native speaker level. Furthermore, I would never get the tone of a syllable wrong, nor make a mistake in my application of the sandhi rules (for the words I know, I must constantly stress).
I guess this is a very interesting phenomenon: the difference between native speaker mastery and non-native speaker mastery. You could have Person A who was born in a particular culture / area, grew up there, and spoke the local language natively up to the age of 10, and then left that culture / area, versus Person B who came as a 25 year-old adult to that culture / area from somewhere totally unrelated. Person A might not be able to converse at all about politics or history or literature, but he/she will never make a grammatical mistake or a mispronunciation, in the things they can say. In contrast, Person B might be able to discuss (even fluently) lots of higher level topics, but might occasionally make a grammatical or execution mistake.
I find that difference and contrast quite fascinating.
... finding out more about my grandparents' speech assured I'd never really feel like learning [standard Japanese] again ...
It's rather interesting to hear of your attitudes and ideas (and their evolution) with regards to standard Japanese and your grandparents' speech. In many ways, there are stark contrasts between that and my own attitude to Mandarin and (Penang) Hokkien.
In my case, I always loved speaking Penang Hokkien. It had a nice warm "homely glow" about it. In contrast, I was always a bit disdainful about Mandarin. Up to my teens, I considered it a "weird and difficult foreign language". The occasional feelings of antogonism between Chinese-educated and the English-educated in my childhood obviously helped to re-inforce this.
It was only as an adult that I started to see the value of learning Mandarin - largely due (but certainly not restricted) to the insights it could give me into Sinitic languages in general and Hokkien in particular.
So it seems to me that we're almost diametrically opposite in our paths in this regard. You started with disdaining your grandparents' variant, and wanting to learn standard Japanese, and ended up valueing your grandparents' variant and losing interest in standard Japanese. I started (and remained) loving Hokkien and disdaining Mandarin; and ended up valueing Mandarin while still retaining my love for Hokkien.