If it helps, in my experience it can be quite difficult to delete or lose stuff on the BT Cloud.
I’ve deleted photos on my iPhone (within the Photos App), then done a sync from the Cloud App Home page on the phone. And the photo is still there in the Cloud.
I then tried getting the photo back by simply downloading it from within the Cloud App Photos. And it ended up back in the iPhone’s Photo App.
Now, I know what you’re thinking. If I then tried doing another manual sync on the phone would there be another copy of the photo I’d just downloaded in the Cloud too?
Well no. Only the original photo (before I’d deleted it off the phone) is in the Cloud.
So it seems I can delete photos on my iPhone but still have them safely in the Cloud. Oh, also, I found that if I edited the photo on the phone those changes didn’t get updated to the Cloud. The original remains.
As I say, this is my experience. It’s best if you try it yourself before deleting loads of precious photos and/or waiting for a Guru/Moderator to confirm.
While I’m on the subject, if you’re on a PC (or Mac I presume?) and looking at the iPhone’s photos within the BTCloud folder…. If you modify/delete a photo there it’ll update the photo in the Cloud (but again, not on your Phone) If you delete it’ll get moved to the Cloud’s Trash folder. If you want it back, just look for it in the Trash 🗑️ and select Restore.
Hope this helps. The main thing is to have a little play around and satisfy yourself what’s going on and be confident that it’s doing what you need.