"truly substantial" is correct and usable in written English.
You can use it to emphasize the fact that something is significant or meaningful. For example: The impact of the new legislation was truly substantial.
It did round out what was a truly substantial meal.
Expect this to be a truly substantial exhibition.
He is impressive but he can't make this character truly substantial or believable.
(Those were the happy days for a nouveau riche, when a teardown meant obliterating something truly substantial).
Bad taste can elevate certain movies to a certain level of popularity, but only truly substantial ones reach the stratosphere.
If you want capital requirements alone to carry the weight of financial regulation, you need banks to carry truly substantial amounts of capital.
It missed a substantial number of truly significant SNVs, while it generated a large number false positive SNVs.
The report concluded the long legal melodrama that resulted in Mr. Clinton's impeachment and sharply split the nation about whether he was the victim of a politically motivated criminal investigation or had truly committed substantial offenses.
A postdoc who is truly "a substantial collaborator" deserves full scientific recognition, Grant says.
We have not made it a high enough priority to truly mobilize the substantial resources of the city to reach the most people eligible".
Many metaphysicians have relied, in this connection, on the internally related notions of substance, quality, and relation; they have argued that only what is substantial truly exists, although every substance has qualities and stands in relation to other substances.
