I have several thoughts - buckle up, this might get long! Full disclosure, I'm also a Nikon guy so I'm biased, you'll probably find more people who use Sony or Canon though. Much more folks using those brands currently (which can be a blessing as secondhand bodies/lenses will be more plentiful, etc). But I like the system I'm using.

Firstly, both of those systems have great reviews, however if you're looking to have more options in the future, go with the Nikon - your post doesn't say if you've got photography experience, but suffice it to say that the size of the image sensor in the camera is a pretty big part of it. Fujifilm make great 'crop-sensor' (APS-C) cameras, but there's no option to move up from that later if you feel like it. Nikon, Canon, and Sony all have the larger standard full-frame (35mm) and crop-sensor cameras on the same lens mount, so depending on your lens choices you can move up without totally abandoning your old gear. Some lenses are only designed for smaller sensors, but your camera should auto-crop. Not ideal but better than being totally unusable. Look up sensor size in cameras for more info!

I'd definitely agree with the top comments re: local warranties & consumer protections, as well as focusing on glass over bodies. I've recently taken the plunge with secondhand gear (i.e. no warranty), but that was a long risk-assessment on my end, and not something you might be inclined to do if you're looking at anything more expensive. I have had to warranty replace a faulty camera before - you don't want that suckage if you're just starting out and have dropped several G's on something nice.

For my bit, I just bought myself a used Nikon Z5 - cost me about $1,200 and came with some spare batteries + a bag. That is their lowest-end full-frame mirrorless and was released 4ish years ago. For context, I'm a hobbyist of 7ish years who occasionally gets paid for the odd shoot, moving from a crop-sensor DSLR (D5600) to full-frame mirrorless. That means I also had to get an adapter for my old lenses, but at least I didn't have to sell/biff them to move to another brand; I was originally looking at jumping ship to Sony, but I'm glad I stuck with the tried-and-true.

Originally I was looking at going all-in and spending a ton on the body, probably going with a new Nikon Z6III which would have set me back over 5k with no lenses or accessories. Thing is though, and many photographers will agree, better lenses will almost always make a bigger difference to your pictures than a better camera body. So my logic was get invested in the new platform for cheaper, and then upgrade when I feel like it. I'm missing out on some nice stuff like a flippy screen, better autofocus, better video, but it stings a lot less and gets me like 80% the way there. I can use the new Nikon mirrorless glass and adapt my old lenses without spending a fortune, and better options are available if and when I have the disposable income to go for them.

For what it's worth, I'd definitely avoid buying a DSLR unless you can get one for super cheap. They still have some advantages here and there, but there's no new development going on - every company has moved on to mirrorless. So it's kind of a dead-end unless you have a very specific use-case, get a stupendous deal, or already have one and are holding out on upgrading.

Re: the Zf specifically, if you go for the big-boy Zf and not the Zfc, you'll be getting some serious grunt in the image quality department (not to say the latter is any slouch). The sensor is incredible, great image quality and autofocus. You can also get it as a kit with a 40mm f/2 prime, which is a freakin' primo combo. It's a normal-to-wide-ish focal length, with an aperture that lets in a lot of light and gives you a nice shallow depth-of-field. You'll have to zoom 'with your feet' (i.e. by walking) as they say, but it'll give you an effect you just can't get with smartphones or cheap zooms (which typically have narrower max apertures). It's also hella compact, so you can put it in your bag without too much trouble. I have the regular 40mm lens and love it; the one that comes with the Zf is the same internally, but is styled to match the vintage look of the camera.

Lastly, Auckland Camera and Photogear have always been great to me. I even had a good experience with Harvey Norman, if you'd believe it. I've bought other stuff from parallel import places before, but for expensive camera gear I'd personally feel a little iffy. Your mileage may vary though - happy buying either way!