The mouse control setup is part of the Accessibility settings as Apple consider it to be an alternate way of 'touching' the screen for those who need it. In other words, it isn't exactly like mouse control on a PC or Mac. You can touch, long-press, drag and so on, but the mouse pointer is a simple circle, representing the end of a finger on the screen, with which you do all of the regular finger movements that iPadOS expects. It doesn't take long to get used to it, but if your fingers work normally it might not give you any advantage using it. For photo processing, as for most things, I prefer the Pencil to either the mouse or a finger.
External storage seems to work well, in the limited way that I've used it. I've copied JPEGs from an SD card inserted into a third-party Camera Kit, but I get raw files onto the iPad using PhotoSync from my PC. I've also set up my PC as a connected server, so I can drag files from the PC to the iPad and vice versa. You can, of course, drag photos onto the iPad's file system, but they won't go into the Photos app, just into a regular folder, so you won't be able to load them into something like Snapseed. On the other hand, long-pressing a JPEG file should give you the opportunity to Save to Photos. I don't know about other photo processing software such as Lightroom as I don't use it, but I believe it expects to import files from the Photos app, not the file system. Perhaps someone else can put me right if I've got this wrong.
In the early iPadOS beta versions, the iPad wouldn't recognise an SD card unless it had a proper eight-character name, but that might not still be the case. I would imagine that they would have fixed this by now.
In my opinion, Apple still needs to do some work on the file system as it's not really a file system in the same way as on a PC, Mac, or Android phone. It's a simulated file system under the control of the Files app. From within the Files app, there's no access to the Photos app, other than copying a JPEG using the context menu. I'd like to see Photos appear as a folder, with subfolders for the albums, so that you can drag and drop directly into and out of them.
If you've used the Files app in 12.4 and below, the main differences are that you can attach external drives and read from and write to them; you can attach a PC or Mac as a server (and probably a NAS, too - I don't have one to try); you can create folders anywhere within the file system and put files there, whereas previously you had to use a workaround such as creating folders within the Pages app to hold your non-Pages documents. These are all very useful changes and if Apple listens to its users and implements some of our suggestions, it can only get better.
