Big Sur Photos App fails to upload all photos from external OneDrive files

Background: I have multiple computers which I synchronise using MS OneDrive. Uploading photos to the Photos App is not an option for me, as I also use third party apps to process these photos. I have both Big Sur (running on iMac 27 2020 & MacBook Pro) and High Sierra (iMac 27 2011 -( a stalwart machine!)).

Uploading some 54,000 photos from OneDrive I note that all three give different tallies! These counts vary by up to ~200 (talk of needles in haystacks!). After many hours, I suspect this, in part, has something to do with processing LIVE photos on BIG SUR, but I simply cannot isolate the problem.

The loading process on BIG SUR is also inconsistent between the instances on the iMac & MacBook Pro. I have managed to upload some of the "missing" photos on Big Sur by manually extracting and re-importing them (thus, demonstrating that it is not the fault of an incompatible photo, or format).

In conclusion, I am confident that Photos on High Sierra is loading ALL these photos, I am NOT confident that Photos on Big Sur is reliable. I won't mention the inability to view the photos' location in the Photos Places feature on Big Sur!

Given this, I would never have sufficient confidence to use this App as a primary storage for any photos. This is a shame as I had hoped my new 2020 iMac would be my primary photos editing m/c - but I now lack all confidence and have reverted to my 2011 iMac!

Any constructive comments would be appreciated - including advice on a replacement App...