What will happen if I turn on iCloud Photos on my iPhone?

My question about Photos and iCloud on my iPhone appears at the end after these background details of my situation.

The Background Details:

 I have an iMac, an iPhone 11 Pro and an iPad Pro.

 Both on my iMac and in iCloud, I keep my complete collection of 30,000+ photos. 

 In Settings on my Mac, my iCloud preferences are set to:

 - iCloud Photos (checked), and

 - Download Originals to this Mac

 On my iPad, Settings> iCloud> Photos preferences are set to:

 - iCloud Photos, on

 - Optimize iPad Storage, checked

 - Upload to My Photo Stream, on

  However on my iPhone, Settings> iCloud> Photos preferences are set to:

 - iCloud Photos, OFF

 - My Photo Stream, on

Currently, I manually transfer photos from my iPhone to my Mac and I sync my iPhone to my iMac to replicate all the albums that I create on my Mac back onto my iPhone.

I also frequently backup of my iPhone to my Mac. The prior sync of the iPhone with the Mac, and the backup to the Mac, can take more than 20 minutes and I believe it has to do with my settings for Photos on my iPhone.

My Question:

If, on my iPhone, I turn iCloud Photos to ON, what will happen?

- Will the 144 photos in my Recents Album on my iPhone be deleted? (It currently functions as the old Camera Roll function in iPhotos).

- Will I create some other problem with missing photos on my iPhone?