Switching husband to his own Apple ID without losing any of our photos

My husband and I have both been using my Apple ID on each of our phones. So all of our photos (over 40,000) are all jumbled together on both of our phones and an iCloud. We want to give him his own personal Apple ID, which I’ve already created online, and are hoping that then his photos that he has taken on his phone will remain on his phone, but my photos will be removed from his phone. Vice a versa, we are wanting only my photos to stay on my phone and his to be removed. Together, we have 40,000 photos and videos that are all mixed in together and on both of our phones and on my iCloud account. Can anyone help us sort this mess out without losing any photos? Thanks so much.

(I also posted another question about how to download all our photos from iCloud to my new 2 TB computer.)