Alternative to all photos from iMac being uploaded to icloud

The problem will be, that the System Photos Library is very special and has two different purposes (System Photo Library overview in Photos on Mac – Apple Support (UK)):

1. First of all, it is the only library that can be accessed from other application, when you want to share photos or use them in another app- a web browser, Mail, or want to save photos to your Photo Library. All this depends on the System Photos Library.
2. Second, it is the only library that can sync with iCloud - iCloud Photos, the Shared iCloud Photos Library, Shared albums.
3. And on our mobile devices we can only have one library.

If you want to use a small library to sync the new photos, this small library will be your System Photos Library. It needs to contain all photos we want to access from other apps, and all photos we want to have on our mobile devices at well. It needs a lot of planning to get it right, so our system photoss library will be suitable for all purposes.

I have written a short user tip on how to use a small, additional Photos Library to replace My Photo Stream: Here's how we can replace My Photo Stream with iCloud Photos… - Apple Community

hazel170 wrote:

My understanding is that if I subscribe to I cloud then all my photos (80 thousand) would upload and be stored there for access.

I have read the article re the suggestions to download to different devices when we wanted a particular photo on that device.

Big Questions !!

Will all my photos stay on my iMac if I choose to store them in iCloud, or will they disappear and I have to download then again to my desktop computer ?

Your photos sync with iCloud and all of your devices. The originals may be on your computer unless you elect to optimize storage. If you optimize, you'll have lower-resolution copies on your computer until you "use" one open/download/move/etc. when the original is downloaded from the cloud. iCloud stores original photos.

Can I choose to not have all my photos stored in the cloud ?

Sure. But, you'll need to create multiple photo libraries (Mac) or move them out of photos (PC). You cannot selectively choose what to sync, it's all or nothing.

I read somewhere that only the photos in the system photo library were moved to I cloud.

Is there a way I can create another library on my iMac to keep these photos on my desktop computer and prevent them from being exported to the cloud storage ?

Yes. See above.