I have samsung watch ultra, and the test photo watch face is working fine. I tap the Edit on phone on watch and it immediately opens the Wearable app where I add photos. Click save and all is shown on phone.
It takes a few moments before the save button available once you selected photos.
Try selecting just 1 or 2 photos max to see if that works.
The photo slot function is not something we control in the watch itself - all you do is add the element, and it is a feature of WFF/Wearable app.
The Samsung/Google style picker is just using Custom TAP actions to open either the Samsung version of Health or Google version. Nothing related to Photoslot though.