The phrase "a quite substantial" is correct and usable in written English.
It can be used to describe something that is considerable in size, amount, or degree.
Example: "The project received a quite substantial amount of funding, allowing for its successful completion."
Alternatives: "a fairly significant" or "a rather considerable".
And this was only the tip of a quite substantial 'bug-berg'.
With your additions of choice, a simple flatbread can become a quite substantial hand-held "dish".
What's more, lots of people are adding to what is now a quite substantial and interesting collection.
Justice Stephen G. Breyer, one of the dissenters in the 5-to-4 decision, surveyed a quite substantial body of empirical research on whether gun control laws do any good.
In the end, they agreed not only that the girl would be without a dowry but even that Rezak would pay a quite substantial amount of money for her, which the family would take in installments.
Saddam Hussein was driven from power, at a quite substantial cost in American, British and especially Iraqi lives and with still unknowable consequences for world and regional political stability.
I use both, and as a shareholder get quite substantial "on board credits" as a consequence.
The court heard Mr Watkins, who supplied cannabis on a "commercial and quite substantial basis", had gone out to meet a drug dealer to "buy a large amount of cannabis".
This is something that would not be been noted for weeks or months so the financial impact of being able to intercept this and action a change was quite substantial".
This position leaves us with a number of quite substantial problems.
In contrast, lithium was associated with a significant, and quite substantial, reduction in suicide death compared to placebo (Odds Ratio (OR) 0.13, 95% CI 0.03, 0.66, p = 0.01).
