I’ve used Seafile for years just for this. I haven’t ran that on pi, but on virtual machine it runs pretty smoothly and android client is pretty hassle free.

What’s your end goal here? You try to keep files just on that one media without any options to make copies of them? Or maintain an image which has enforced files at their directories? And against what kind of scenarios?

ACLs and SELinux aren’t useful as they can be simply bypassed by using another installation and overriding those as root, same thing with copying. Only thing I can think of, up to a degree, is to use immutable media, like CD-R, where it’s physically impossible to move files once they’re in place and even that doesn’t prevent copying anything.

Geeqie is a quick one to go trough photos and it groups RAW+JPG as a single item on preview, so even a few hundred photos are quickly ran trough with just a keyboard. I’m not sure on how well it manages tags as I don’t use it for tagging, but it’s most likely in your distros repository so testing it out is quick.

Then do sudo apt install xfce4 and sudo apt purge cinnamon* muffin* nemo*.

It’s been a while since I installed xfce4 on anything, but if things haven’t changed I think the metapackage doesn’t include xfce4-goodies and some other packages, so if you’re missing something it’s likely that you just need to ‘apt install xfce4-whatever’. Additionally you can keep cinnamon around as long as you like as a kind of a backup, just change lightdm (or whatever login manager LMDE uses) to use xfce4 as default. And then there’s even lighter WM’s than XFCE, like LXDE, which is also easy to install via apt and try out if that works for you.

I understand the mindset you have, but trust me, you’ll learn (sooner or later) a habit to pause and check your command before hitting enter. For some it takes a bit longer and it’ll bite you in the butt for few times (so have backups), but everyone has gone down that path and everyone has fixed their mistakes now and then. If you want hard (and fast) way to learn to confirm your commands, use dd a lot ;)

One way to make it a bit less scary is to ‘mv <thing you want removed> /tmp’ and when you confirmed that nothing extra got removed you can ‘cd /tmp; rm -rf <thing>’, but that still includes the ‘rm -rf’ part.

Absolutely. Maybe leave Gnome/KDE out and use a lighter WM, but they’ll be just fine. Specially if they have 8GB or more RAM. I suppose those have at least dual core processors, so that won’t be a (huge) bottleneck either. You can do a ton of stuff with those beyond just web browsing, like programming/text editing/spreadsheets and so on. I’d guess that available RAM is the biggest bottleneck on what they can do, specially if you like to open a ton of tabs on your browser.