'collectiveness' is a correct and usable word in written English.
It refers to a quality of being unified and belonging together, and is often used to describe a group of people. For example: The collectiveness of the team was evident in their ability to work together to achieve a common goal.
Dictionary
collectiveness
noun
The state or quality of being collective.
synonyms
Exact(4)
Gustavo Arellano, the writer behind the widely popular syndicated column "¡Ask a Mexican!", said it is through the process of making tamales that culture is communicated across generations, encompassing the value of family, nourishment and collectiveness.
I discovered a joy and security in the collectiveness of shared experiences.
There is a real collectiveness here.
Why? "Because 'soldier' is restricted to ground troops and has implications of one who simply does what he is told and since 'troops' in its very collectiveness precludes individual heroism.
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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