I've been basically iPad only for over 2 years. I have an 11" iPad Pro (2018), and it's great. I can't imagine ever going back to sitting at a desk and editing with a mouse now. I only use LR, the iPad version of PS is terrible but Affinity Photo is apparently a very good PS alternative and works very well on the iPad. Since the LR workflow on the iPad is very heavily cloud storage based you should be ok with 128Gb. I've got a 256Gb iPad and it's nowhere near full, and I have access to my entire LR catalog with 12 years of images. One thing to factor in is the cost of the Apple Pencil. It's expensive but essential for photo editing. The lack of LR Classic is no issue at all, unless you do a lot of printing, HDR or Pano merging. Those things are missing from the iPad app, other than that it's fine. The iPad LR app (and the desktop app actually) make LR Classic feel like something from the last century.
For those saying the screen is too small, it isn't because you're sat a lot closer to it, essentially holding the image in your hands while you're editing. That said, when I eventually replace this iPad I will go for the 12.9" version.
If you want to save some cash the Air will be fine. You lose a couple of speakers, a 120Hz screen and a couple of other bits and pieces but functionally it will be identical to the iPad Pro. File management is absolutely fine, I can complete my entire editing, exporting, WordPress and Portofolio workflow solely on the iPad. iPadOS has its quirks but the main issue is that people expect to work exactly like MacOS / Windows, and it doesn't. But a lot of the stuff you'll see online about file management being impossible and all that simply isn't true, you just need to learn how the OS works the same as any other device. It could definitely be improved but it's far from the horror show some articles make it out to be