I just got a new iPhone 11Pro. LR Mobile got hung up because it was trying to download everything from either the Adobe Cloud or move over the Apple photos. I've now gone over my data limits on my home WiFi and my iPad also seems to have downloaded more photos too. I have 27,000 LR photos on both and I don't need all my photos on both devices. I stopped syncing with Adobe Cloud and I stopped automatic Apple Photos to LR Mobile on my iPhone. I still seem to be using too much data. Any suggestions on how to get back to the settings that were working for me previously? Besides this forum where can I go for help?
Lightroom on your phone is not syncing down the full data set for your 27,000 image catalog. It will sync down account data, and thumbnail images, and then for images you choose to edit with will download smart previews (which you can think of as high res but not full resolution copies). You can also tell Lightroom to download the original image, and these can be large... But this isn't happening for every image, just the ones you select.
Unless you are importing thousands of images from your iPhone camera roll into Lightroom, or unless you are enabling "Store Locally" for lots of albums, LR on your iPhone really should experience reasonable data use. Do check out the help page above for more info. Another great page is the Getting Started article: https://helpx.adobe.com/lightroom-cc/how-to/lightroom-mobile.html