http://www.holocaust-history.org/auschwitz/body-disposal/
Looks fairly even-handed and informative.
I have the book “Babi Yar” by Anatoli Kuznetsov who was a child in Kiev during the Nazi occupation. It’s a great read.
I don’t know if it’s still in print but the copy I had was in three different fonts. One was the original type, another bolder type showed what the censors cut when it was published in the Soviet Union, then another font to show parts he added after he defected.
You’re underestimating the capacity of the system. Hiberg’s final numbers for the number of bodies the system was able to “process” are substantially higher. By death camp:
Auschwitz 1,000,000
Treblinka <750,000
Belzec 550,000
Sobibor <200,000
Kulmhof 150,000
Lublin 50,000
Aside possibly from the latter, this is significantly more than tens of thousands per year per camp. But they managed it.