I wish there was an easier way to add additional photos quickly. From what I understand the only way is search for the app in settings, open the "Photos" permission and click on "Edit Selected Photos". Is there an easier way I missed? I guess I can't expect a simple click from within the app itself while selecting photos, as it's not aware it's seeing a limited selection.

I think it requires developers to fix that. At least in Slack there's two buttons next to their in-app image picker. One opens the camera to take a photo, the other pops up the iOS photo picker overlay where you can select more images.