I am writing a sentence as below and am having issues with plurization of a phrase (highlighted).
The Cluster had begun as a research group, a gathering of the sharpest minds of the world. Proponents of progress, the group was a force to be reckoned with.
Microsoft Word's grammar check seems to find the above sentence invalid and has provided the below two suggestions but both of them feel strange to me.
A proponent of progress, the group was a force to be reckoned with.
Proponents of progress, the group were a force to be reckoned with.
My understanding is that in the beginning of the sentence I am referring to a group of people so it should be plural and in the second part of the sentence, the focus is on the group as a whole so it should be singular. Is my understanding correct ? If not, what sentence structure would be grammatically correct ?