Wondering if I’m correct about this (はvsが)

I wrote the following text to a friend explaining my understanding of は vs が and I’d just like to know I’m not completely talking out of my ass and if my understanding is correct or not and if not what is wrong about it

I’ve been trying to put into words my understanding of は vs が and I think I’ve pretty much figured it out

Here’s my example
So obv は topic marker が subject marker

Ex:
waiter:コーヒーのみますか？
Would you like coffee?

が example
You: ジュースがありますか？
Do you have juice

は example
You:ジュースはありますか？
(As) for juice do you have?

Now which one is correct?
For a sentence to be grammaticaly correct it has to have a subject, in the は example it is omitted but by context we can infer it would be
のみものがジュースはありますか？
This is hard to translate but I’ll do my best
Drinks is the thing I’m talking about and as for juice do you have?

Now that translation didn’t just sound wrong it is grammatically incorrect therefore the correct would be the first usage が
(飲み物は)ジュースがありますか？
(As) for drinks do you have juice?

Then of course there’s the good ole
好きな動物はなんですか？
“what kind of animals do you like”
私は犬。
“as for me dog”
Vs
好きな動物はなんですか？
“What kind of animals do you like”
私が犬
“I am a dog”