Thanks Ger. Yes, I have rebuilt hundreds of workbooks from scratch exactly as you've described. In this situation it's ~8 hours, because the named ranges and time it takes to rebuild the calc trees. However, it 100% doesn't fix the problem, the files keep getting corrupted upon saving (they never crash while I'm actually using them). I'm still willing to pay someone thousands of dollars to find the real problem.
I have dozens of very large MySQL databases on AWS - I'm not using spreadsheets to warehouse data (and Ctrl + End takes me to the end of my formulas). My spreadsheets are as efficient as can be while still performing their function (financial analysis & backtesting).
Lastly OneDrive has been pretty solid - syncing ~600GBs across 4 PCs throughout the country. It has been much better than Google Drive for me.