I only rarely need to work with Macs, but Apple rewrote its SMB implementation sometime in the 10.12 or 10.13 timeframe (to avoid GPL licensing of the existing open-source one, I believe) and it's been terrible ever since. Even the OSS one wasn't great but I remember reliability was much improved when forced to CIFSv1 (the original simple-but-insecure protocol) than when using any of the newer versions.
...I suppose anyone who has the time to work on this problem could find the last open-source release that Apple used, and port it to the newer versions of macOS.
...I suppose anyone who has the time to work on this problem could find the last open-source release that Apple used, and port it to the newer versions of macOS.