You can have several Photos Libraries on your Mac and only one of them can sync with iCloud Photos.
So you could keep all of your Photos in your current Photos Library and create a new, empty library for syncing with your iPhone. Then you need only enough iCloud Storage to hold all the photos you want to keep on your iPhone.
See: Create additional photo libraries in Photos on Mac – Apple Support (UK)
And you need to make the new library your system photos library, before you can enable iCloud Photos for it: System Photo Library overview in Photos on Mac – Apple Support (UK)
I am keeping in my iCloud Photos Library only the new photos I am currently working with, and plus my most favorite photos I want to have on all my devices.
As to the price of of the 2TB iCloud storage plan - the gap is not so large as you think. Originally the largest plan used to be just 1TB, and Apple increased this to 2TB, without increasing the price.
iCloud Photos has advantages compared to My Photo Stream - it can transfer all media from your iPhone. With My Photo Stream we could not transfer the Live Photos or videos, and the depth maps of Portrait Photos have been missing, and the photos have been transferred right from the camera, before we could add adjustments or metadata. Any modifications we applied later have not been transferred automatically. With iCloud Photos the library on your Mac will always be identical to the library on your iPhone. You can organize the library on your Mac and enjoy the same albums and folders, captions, adjustments on your iPhone or vice versa.
