iCloud issues between iPhone + Photo app on MacBook

Q1. Yes. Enable iCloud Photos on your Mac. Open photos. Then photos, settings, iCloud. Your photos will now sync automatically. Remember it’s syncing so any changes anywhere are reflected everywhere.

Q2. If you disable iCloud sync on your iPhone you’ll be prompted to download images that have not fully downloaded. This is especially true if you elect to optimize storage (settings, photos) as space optimized versions, not originals, are on the device. If you turn off syncing whatever is on your device remains.

Great. You can still sync manually. But, as you currently sync using iCloud stopping that sync means that you either need to delete the images on your device or download them all from iCloud.

essentially you start fresh. It’d you don’t have space on your phone to download the entire library, you’ll need to delete the images currently in your device instead. You can then manually sync them back.

if it were me, I would

First sync my Mac to iCloud and download all of my images that reside in iCloud.

second, ensure that the photos currently on my phone are in iCloud (already synced)

then turn off iCloud sync on my devices. Keeping the library on my Mac as the “source of truth” and deleting the images on my phone

finally, sync the phone to your Mac to sync whatever photos you wish to keep on your device.

all future syncing would then be performed manually.

and, be sure to use Time Machine to back up your photos

Thanks for the clarification. If you stop syncing with iCloud, you can then sync again using your Mac. You'll be able to sync individual items, individual albums, or everything so you can select what you wish to have on your phone. Provided the items on your device have already been synced through iCloud you can sync them again using your Mac. iCloud sync is all or nothing. Syncing with the computer allows more granularity.