I asked a similar question and I was recommended a FujiFilm X-T3, X-T4, or X-T5. Several people told me that the Fujifilm mirrorless cameras are good for learning to do things properly, with well-placed physical dials, and they still work pretty well when you just want full-auto mode. I got a used XT-4 with an 18-55 lens for well under $1000. As a beginner, I am happy with it.

I then bought a Tamron 18-300 lens, which is about $700. Professionals may scoff at it, but as a hobbyist, it is great - it is fast enough over such a wide zoom range that I can carry just one lens, and whip it out and take pictures fast.

A wise man once told me that the best camera for you is the one you will take the most photos with.

First and foremost... I would check into buying used from a reputable reseller that guarantees their equipment:

https://usedphotopro.com/

https://www.mpb.com/

https://www.keh.com/

IDK where you're located geographically, but a couple of those do ship to different countries so they should have you covered.

As for your specific camera needs, it's going to depend on the setting you're taking these pics in. It's less about subject (nature, graffiti, consenting butts) and more about speed and and light levels.

You're right that taking a frozen moment from a moving vehicle you need a fast shutter (this is different from a "fast lens"). Most modern digital cameras should have no problem getting a fast enough shutter speed... something that tops out at 1/2000 or 1/4000 should be plenty. The farther away it is, the easier it will be.

Nature pictures like landscapes are generally pretty easy to shoot. They don't move around much so you have plenty of time to set things up and find your scene. Nature pictures like animals... that's a whole other story and takes specialized stuff usually, depending on the animal. The good thing is that with interchangeable lens cameras, some of that is just the lens and can be upgraded when you figure out which niche you're trying to fill.

When you get into these types of cameras, you are really buying into a lens system... that is a good place to start thinking about it. Will you be able to get lenses you like for a price you can deal with. Often that means not only how are the first party lenses, but are there good third party lenses. So take canon for example... great first party pro lenses, crap entry level lenses. Their DLSR has access to stuff like Sigma which are really good... their mirrorless they more or less banned 3rd party so you're sol. So a little research on that is a good idea.

So my suggestion, personally, is to look at micro-fourthirds flagship cameras from the last decade and flagship DLSR as well. You will get a lot more bang for your buck and quality from an older flagship pro camera for the same price as an entry level new camera. There is a hidden price advantage for M43 cameras as well... the lenses are cheaper, lighter, and since it's an open format there's lots of choices from multiple manufacturers. If you get an Olympus camera, the lenses work on Lumix. If you get a TTartisans M43 lens it works on both. So you have a lot of choices for very inexpensive. Basically all M43 cameras have In Body Image Stabilization as well... which is really nice for getting rid of camera shake while taking handheld shots in lower light.

As for software. There is plenty of stuff that will work well on linux... and lots of them are likely in your repos already or have flatpak or appimage. I really like RawTherapee for processing RAW images. https://www.rawtherapee.com/ There are some good scripts to get Affinity Photo working on linux these days as well such as https://github.com/ryzendew/Linux-Affinity-Installer ... just read thru the info thoroughly, but it's basically run shell script and click buttons on the GUI.

On the train you will want a fast lens, meaning low f-numbers (takes in more light) so you can use quick shutter speeds without it being to dark.

You might also want a camera body and/or lens with image stabilisation. Not a must, but good if shooting on the move or using long shutter speeds handheld.

Sorry for ignoring your buy nice, not twice. But you don't know what you want until you get your hands dirty. So get something less expensive and learn. Then buy someting nice.

I recommend either a cheapo used Nikon APS-C DSLR (Nikon D3200 for example) and some lens to go with it. Could cost ypu about 150€ to 300€ to get started depending on how lucky you are.

Or get a used mirrorless with a lens for a fancier start. A bit more expensive, but probably better than an old DSLR. More likely to have image stabilisation as well.

Then if you like to shoot, upgrade.

Compare different cameras on https://cameradecision.com/

For image processing I revommend Darktable. FOSS and Native to Linux. I've only used that and it works well for me.

PS.

I don't know what hardware to get for the butt pics. Maybe a wide angle lens if it's a big one and you like to get up close.

Not my area of expertise though. Someone else is welcome to chime in.

@ArcaneSlime see what you can get your hands on locally second hand/used. See what you like the feel of.

You should Look into Micro Four Thirds, theyre very affordable and great https://youtu.be/oVqE6Js__VU

Otherwise just get an old DSLR from like 10 years ago via mpb

For RAW editing you can use darktable

But good luck finding a camera with open software, maybe a PiCamera ?