Full frame is overkill for a beginner. Lenses are much more important than bodies or format. You're much better off getting a second-hand APS-C body from a few generations back, and invest in lenses, than getting a top FF body and just a kit lens.

Depends on the budget of course. But you'll want to experiment and try different lenses, so that's what you need to budget for.

Which system/brand is the best - neither, it depends on your preferences.

First you should decide whether you want a DSLR (with an actual mirror) or an EVF camera. That's pretty important for the feel, and it's to your preference.

Second is the ergonomics of the system. Fujifilm and Olympus (or what's that they call themselves now) generally regarded as very "classical", Canon, Nikon and Pentax have their own styles that have evolved over 70 years, and Sony and Panasonic are more modern and more video-focused.

And then there are lenses. There's quite parity between systems so it doesn't matter that much which you choose, with the exception of Canon M system which has weak support and is dead. So the M50 is good only if you can get it cheap and treat it as a one-off for learning. (Ed: actually I think you can mount adapted APS-C DSLR lenses so there's still an upgrade path.)

I used to have Nikon DSLRs, but I'm not a huge fan of their new EVF offerings.

Sony is the biggest brand out there but I never liked their ergonomics and style.

Canon is kind of a gold standard, but always make weird choices re what can be in which model, and lately seem confused.

Pentax probably has the best ergonomics and controls ever, but is an extremely niche brand at this point.

OM system is lovely, but their smaller 4/3 sensor makes lenses different and is on life support.

If I was starting from scratch today, I'd probably go with the Fujifilm X system. Lots of various choices, been around for a while, good support, open lens system.

I suggest you look at Thom Hogan's sites bythom.com and sansmirror.com

My case is different from yours as I focus on street photography, but I recently got my first camera, an a6400, and I'm extremely happy with it. Took some ~30k photos so far.

I wouldn't start with full frame unless you have a lot of money to spare. The lenses are much, much more expensive, and if it's not something that sticks you'll be throwing a lot of money away.

At this point in time, people like you and me are still figuring out what kind of photography (and thus equipment and lenses) we like, so I think it's better to experiment with less expensive gear before committing.

Whatever you choose, please make sure you figure out the ergonomics.
I have an a6100 and I find it big and heavy when I hike with it.
I recently started carrying it on a capture clip on my belt and it made it significantly better.
Don't forget to check the lenses too. Weight, price, size.

Find a camera that you like to use.

35mm isn't the be-all and end-all like some like to say. I really wish this mentality would go away.

Your budget needs to include some lenses; the other comment about lenses is pretty much spot on. No point spending so much on the camera when you can't do what you want because you're now stuck with limited lenses. You didn't say what you want to shoot, so that could cover very broad subjects, from insects to birds to landscapes (a full high end kit to set that up at the 35mm end can easily be 4-6kg).

As sensor tech hasn't advanced that much in the last 10 years or so, it's better to get an older higher-end camera than a budget new camera; the controls and UI tend to be better, which will be better for you to learn photography with. Budget cameras tend to cheap out on controls, so having to menu dive to make basic setting changes will get old very quick.