Hmm, I wonder if a double pagination would remedy the problem and be the best of both worlds. Pagination before cascade, and pagination after cascade. I am not sure how to easily convey this but maybe a query would look like:
{
var_a as var(func: eq(<qa.type>, "Device"), limit: 1000, first: 12) {
...
limit being the new suggestion. Then you could add a limit pre the cascade and a pagination post the cascade. This still would not solve all use cases but would be better then iterating over 6M nodes.
To define limit: To limit the galaxy in queries using pagination with cascade.