What does the "over 8m fewer people" mean?
Solution 1:
It means the new value is equal to the old value minus 8,000,000 and further minus an additional positive integer number, X, where X is likely to be significantly smaller than 8,000,000.
The original sentence is unambiguous. The proposed alternative sentence:
People in work are fewer than before the pandemic, the number of which is over 8m.
is ambiguous, as it isn't clear if it refers to the number of people in work, or to the magnitude by which the number of people in work has been reduced.