irobbig wrote: ... This Mac I am on though does not have the capacity to take in the online library, that is for sure.
Well, the best thing would be to use an external hard drive formatted in either APFS format or Mac OS Extended (Journaled) format for you Photos Library.
If you connect your Big Sur Mac to iCloud, then all of its pictures will be copied to iCloud Photos. You need to make sure you have enough room in iCloud storage for this. Then all the pictures iCloud Photos will be copied (eliminating duplicates) to your Big Sur Mac. You have to have enough space. One way out is, on the Big Sur Mac, to set Photos Settings>iCloud to "Optimize Storage." This keeps only smaller images on the Mac, relying on iCloud to provide the full sized images when you need them. The Photos Library may take up less than 20% of the space that the full Library would use.
The downside to "Optimize Storage" is that, without having the original full sized images on your Mac, you can't back up your pictures. iCloud provides limited backup--if your Mac gets run over by a truck, then the pictures will still be in iCloud. But if you accidentally erase a bunch of pictures (it's amazing how often this happens,) then the erasure will be copied to iCloud, and there goes your backup. So we like to have "Copy Originals" checked instead of "Optimize" on our main machine.
Again, your best bet is to get the fastest external drive you can afford, and put your Library there.
Move your Photos library to save space on your Mac - Apple Support
And then you should keep another drive with a Time Machine backup. Photos are precious!
