OK here's my plan of action. Open to advice/suggestions.
1. Format my new SSD as APFS. Copy data from my old HDD to the new SSD.
2. Back on the Windows desktop, reformat the Toshiba (still NTFS) and copy over the data from the second HDD in the desktop and put it also on the new SSD on the Mac.
3. Once all the data of the old desktop drives is on the new SSD, I can upload the data to the cloud.
4. Reformat the Toshiba to exFAT and use it as a backup drive for the SSD.
My Mac mini M1 only has 256 GB ssd inside. I am considering directly using the data on the external drive to work on. Is that safe enough since I am using APFS? Or should I copy files on the Mac internal storage to work on and put them back on the ext SSD after I'm done? The second option will require a strict discipline on my part.
Freefilesync is available for macos. I suggest to first experiment with test folders & dummy data to understand how such software works as choosing wrong setting/option may result in accidental deletion of data. Once you are familiarized with software interface & options then use it for your actual folders data tasks.
Download FreeFileSync 13.5. FreeFileSync is a free open source data backup software that helps you synchronize files and folders on Windows, Linux and macOS.
freefilesync.org
Thanks. You are a big help.