The phrase "was fundamentally" is correct and usable in written English.
It can be used to describe the essential nature or core aspect of something in the past.
Example: "The theory was fundamentally flawed, leading to numerous misconceptions in the field."
Alternatives: "was essentially" or "was intrinsically".
Dictionary
was fundamentally
adverb
To the very core of the matter
synonyms
Exact(60)
Saul was fundamentally sound.
Next, spying was fundamentally illiberal.
The A.C.A. was fundamentally different.
But his argument was fundamentally flawed.
"But I knew I was fundamentally Chinese".
The secular Zionist dream was fundamentally democratic.
Similarly, eliminating Glass-Steagall was fundamentally sensible.
For me the change was fundamentally important.
Moholy-Nagy was fundamentally a painter.
Badaró's process was fundamentally no different.
He was fundamentally opposed to religious toleration.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com