And I say this as someone fully (and at times exasperatedly) embedded in the Apple ecosystem; if you have lived thus far without it, stay clear.
If I could reset I would go Android + Windows. The options are far greater, you’re not hamstrung by Apple’s world view and nowadays there is plenty of excellent hardware that is equally as swanky as the Apple kit.
I bought a MyPasport Wireless Pro Portable Harddrive this time last year for the exact purpose your looking for (My iPad Pro 12.9 Second Gen) - its a 4TB (there's 2TB/3TB options too) - a SD Card Slot and a USB Slot - it will do a quick backup of an SD Card - stick the card in and press a button.
I used this for storing Movie Files and Photo's on and then can access them via an app called FileBrowser Biz as you can add various external sources like Wireless Drives, etc.
It really only works as a backup though - I did however find that using the Apple SD Card Reader it was easy to import RAW's to Apple Photo's and then import them into Lightroom for culling, tweaking, editing (including in Affinity Photo) - albeit a bit of a faff.
It would be much easier if Apple just allowed you to plug a drive into the USB-C / Lightning Port and crack on...
Its one of the reasons I ditched my iPad Pro 12.9 - I did try the new one (picked up a 512GB on launch day) but as much as its a great device I just thought that close to £1800 for the Device, Pencil and KB Case was getting a bit nuts given some of these types of limitations - I do miss the simple ease of it v a Windows device though.
A hard drive is only for storage, it is where software and user files live. For processing photographs, it is the processor (CPU) and the memory (RAM) that you need. Most later generation iPads have good CPUs and RAMs that are capable of processing photos so you should be fine using an iPad (regardless of how much storage space) for processing photos. The only problem is that it only depends on the App itself. Usually an iPad app will have limited function compared to a desktop based application software having far more tools.
You have to remember that the 256GB storage ("hard drive") is not really going to give you 256GB. An iPad storage will be taken up by the iOS, any installed apps you have, and any other data you have, but usually depending on what apps you have and how much data you used, vary like tens of GBs, still leaving you with hundred of GBs of storage space. (Let's just say purely as an example: Of a 256GB storage space, you could be using 30GB for iOS, installed apps, data for the other apps (ie contacts, email, etc.,) and still leave you with something like 220GB for photos.)
I have an iPad with 128GB and it used up around 44GB but that's mostly for iOS, about 50(ish) apps, about a hundred of books for the iBook app, and some photos, plus mostly user data. All of my main photos are mostly on my desktop computer and sync to Creative Cloud rather than all on the iPad.
And remember: You do not have to be limited to Adobe's Creative Cloud. Nobody says you have to use Brand-A's own cloud storage or Brand-B's own. You can have any and as many of them. For example, I have Adobe's CC storage, Microsoft's OneDrive, Apple's iCloud, and Dropbox.
For proper RAW editing there's only really two big games in town - Affinity Photo which is fantastic and I found able to replace Photoshop.
Lightroom via the CC Subscription is great for cloud syncing and managing photo's, and the new iOS Photoshop (which is being described as an iOS equivalent of the desktop Photoshop) is due this year and should be fantastic!