Office Pro w/ Excel 2019 64bit

I had an i5 with 32GB and Excel would not use more than 15% of the memory
I now have an i9 with 64GB and Excel will not use more than 10% of the memory

These are ridiculous files with millions of formulas on over 100 tabs.
saved as binary
Events = False
Calc = Manual
no Addins
nothing else is open
Recovery is disabled
Recent WB down to 5
no event macros
I've tried it all - Indeed & a sledgehammer are next

Coworkers using the same files on Office 365 aren't having the problems I have.
(One did until he switched over to 365.)
My old personal laptop with i7 and only 8GB can open these files (although it takes a minute!)

Why am I getting out of memory error at 10% memory usage?

I paid for 2019 I don't want to have to pay again for 365.

Please don't tell me to divide the files, paste values, etc. These aren't my files they are my employers. Modifying them at the current time is not an option.