If it was any other country your argument would be substantive. But we are talking about Israel and Palestine. Anyone who leaves will not be able to return. That’s how it’s always been. And for the Palestinians, their land is their existence. They will not give it up. They know the second they leave, it’s been lost.
You’re absolutely right that there will be variance, and that each side has incentive to inflate death counts, etc. Other orgs will likely not produce their own numbers until after this mess is over. Check the Wiki pages or B’Tselem pages on previous assaults on Gaza to see how the numbers can vary, that’ll give you an idea at least.