I use iCloud Photos. My iMac and MacBook get the original versions, my phone and iPad get optimized versions. When I take a photo on one device, it appears on all devices, including the cloud without needing to do anything. If I purchased a new Apple device, I would just log into the Cloud and it will re-download all my photos (been through this lots of times). If I were selling a Mac, I'd sign out of everything and probably format the drive just to be safe.
So, what is it about that plan that doesn't meet your needs? Are you just unaware that you could do that in the first place or is there some specific reason you've not shared why that won't work (for example you might have a large library and little iCloud space).
FYI, in case you're curious, my photo library is almost 33,000 photos and 500 videos. Looks like it's taking 127.7GB on my MacBook and 6.09GB on my iPhone. So choosing optimized versions on your other devices doesn't take much space if you ever wanted to go with that setup.