Thanks Godfrey. I would be interested in learning more about your workflow, and in particular how to get selected images from the SD card on the IPad. I "only" have a "normal" IPad and not the Pro version but I don't think that makes a huge differences. Mine has 64 GB, and that should be sufficient if you only transfer selected images.
I've been working with the iPad for casual image processing since the iPad 2 with 64G storage: it's eminently possible to do image processing with all of them. The key difference is performance for raw image conversion, the non-Pro systems generally are too slow and unresponsive to be proficient at it for more than the very occasional conversion (Pro models have both more processor and much more RAM available, both of which are incredibly important to raw conversion processing).
It doesn't matter much, however. I used to always set my cameras to JPEG + raw storage and get the JPEGs out of the camera as close to the mark as possible, then the post processing on the JPEGs requires only a light touch and is very responsive. Nowadays, I simply use DNG only (my M-D isn't capable of anything else) and with the iPad Pro the performance is quite satisfactory.
So, there are at least two parts to an iPad image processing workflow: moving the image files from the camera to the iPad, and then editing and outputting the results.
Moving the files to the iPad can be accomplished in several ways. I use two of them, depending on what I'm trying to do and on my patience at the moment. With cameras that have WiFi remote iOS app control capability, like the CL, I connect the camera via WiFi and review the latest sessions shots live off the card. I download the files (raw or JPEG, or both, depending upon capture settings) usually individually. The app puts them into the Photos library on the iPad, typically in the Camera Roll and into a Leica folder or album. This works for a quick move of a few images out of several dozen (my usually shooting session).
The other way I move images to the iPad is with the Apple Lightning to SD Card Adapter: It works very simply. Just take the card out of the camera, fit it into the adapter, plug the adapter into the iPad, and Photos runs automatically and the Import screen comes up. You select the images on the screen and then press the "Import" or "Import Selection" button, and it moves the JPEGs and raw files all into Photos. The problem with this is that the selection mechanism is very very slow to render the thumbnails with raw files ... So what I tend to do rather than wait for it to render all the thumbnails is just say "Import All" and let them all flow onto the iPad. It will only import the new ones that aren't already in the library, and it puts them into the Last Imported folder as well. At the end of the Import operation, you are given the choice to remove or keep the files on the SD card ... I always keep them. Once done, just disconnect the Adapter and put the card back into the camera.
To keep from filling up my iPad unnecessarily, I swipe through the images in the Last Import folder individually then and mark the ones I want to keep as Favorites. Once I've done with that, I go back to the grid view and can see the favorites marker next to the thumbnail image. It's easy to select all the others and then hit the Trash button, leaving only the ones you want to work on. I leave them set as Favorites (so I can get to them in the Favorites folder) until I work on them, and then unset the Favorites flag.
You're now ready for
image processing.
There is a plethora of apps for iOS that both operate the built-in camera and can access and edit image files in the Photos library ... in addition to Photos itself. I have been using Photos and Snapseed (which does very nicely with JPEG original files), and am now learning Affinity Photo (which is much more complex and also does an even better job). All three will read files out of the Photos library, both JPEG and raw, and process them. Each has its interesting advantages and foibles, and very different UI. I occasionally use other apps as well (like ShakeItPhoto, Polarr, etc) for their specific effect capabilities.
The image processing workflow is the same as what I typically use with Lightroom in concept ... Selective edits to balance the tones and colors in an image, global edits to raise or lower the total brightness/contrast/etc, sharpening, effects, framing, and all that ... and each iOS app has its own take on the UI and workflow required to accomplish this. That's something that you simply have to explore and figure out what works for you.
-- Note that Affinity Photo is a MUCH richer and more full featured image processing app than nearly any of the others. It is complex, like having a mini Photoshop on the iPad, and I'm still getting my head around it; but I'm making some headway in understanding it now. It produces the best raw conversions I've seen so far on the iPad so it seems to be worth the effort to learn. --
Most apps either modify the original file in place or provide an editing instruction layer. I usually duplicate the file in Photos first (so as to ensure that I can return to the original no matter what pathological thing I do in an app) but then export from Snapseed into a new file with all my edits in place. The exports are placed in Photos and are then accessible to any other apps use (sending, messaging, etc). With Affinity Photo, I'm still learning how to export from its environment and haven't found the way to put the output into Photos directly for the same capability ... but I'm sure it's just a matter of not understanding yet.
Ideally, the same instructions for all of photography apply: try to get things as right as possible in the capture first, and the image processing and sharing becomes easy after that.
G