Apple should improve the UI of this photo selection because it’s very cumbersome to scroll and select the same photos twice.

Agreed. The feature set is in desperate need of the search option both on approved photos and when attempting to approve additional photos. Very often I have to go into the photos app, find the photo, make a mental record of approximately where it is in history and scroll scroll scroll. Obnoxious and cumbersome.

What I really want is to create a special photo album for (Facebook/Instagram/Slack/etc.) and have it automatically gain access to whatever photos I put in there.

I think they have because with chatgpt you click the photo icon and it uses the system photo picker to pick a photo. I guess Meta deliberately isn't implementing that