I'm using a dashcam as a source for pictures. I can't say anything about the one you're interested in, but I have an iTracker GS6000-A7 and it works fine! I'll write down what I do to upload the dashcam pictures:
To upload from the dashcam, I take out the SD Card and put it in my PC and transfer all files onto it. Then I open a program called RegistratorViewer 6.0 (from iTracker homepage) which lets me display and extract the GPS route for specific videos as a GPX file. Also, since dashcams usually record in 3 or 5 minute intervals, it collects all videos from one route to give an overview. For example, if videos 0007 to 0011 are one route, I take these five video files into a separate folder (personal decision to keep things clean) and extract the GPX file into that folder too.
Now comes the more inconvenient part: I take my video editing program, import all five .MOV videos from the folder and save it as a new video: 0007-0011.MP4. The conversion from MOV to MP4 can't be avoided unfortunately, as Mapillary only takes MP4 videos. But this doesn't take too much time, it's just an inconvenient extra step. If your dashcam records in MP4, this can be skipped of course. But I still think you'd have to bring all single video files of one route together as one video, as the GPX file is per route, not per video (meaning you have a GPX location file for 30 minutes of driving but 6 video files each of 5 minutes length)
When that's done, I upload the video and the extracted GPX file here, make sure it's synchronized and hit upload. Once that's done, it takes one or two days to process the video and your images are published.
So to sum it up, it takes some extra time on my PC, but since I don't like mounting my phone every time I want to record while driving I like this variant more.
I can't promise it's as easy on other dashcams (the Registrator Viewer for example is only for iTracker dashcams) but it's definitely possible.
I hope it helps you figure out what's the easiest method for you personally. If you decide to use a dashcam though, I'd recommend a iTracker one, just because of how easy the GPX extraction is. Because if the dashcam doesn't just place the GPX route into the folder it can be hard to be extracted on some dashcams. Also, at least on mine, you can disable all HUD elements like speed, date/time etc, like I did.
What I also noticed is that the video processing by Mapillary seems to only take very short intervals for images, so that you only go very little steps by clicking "next image" when viewing the images. See an example by me here. But that doesn't necessarily has to be a bad thing. Also, the processing makes the image quality worse. If you zoom in on my example I just linked you can see that the quality is pretty bad. The videos have had a better quality on my PC. So keep that in mind in case you want to record something that's worth having a more detailed/zoomed view of.
It has wifi, so I was hoping a direct upload might be possible, somehow.
I highly doubt that. It just makes transferring files to your PC easier I think.
Does the camera write over old data automatically?
Dashcams do this by default, yes. You can usually turn that off though so if the SD card is full it just stops recording.
My 64GB SD card lasts for 7 hours of recording at 2560x1080 and 30FPS. Same for 1920x1080 at 60FPS. It displays how much time is left so I take out the SD card and copy all videos to the PC once I only have 3-4 hours left. So unless you want to record 7 hours or more until you have access to your PC again, it's fine. But to be safe I usually take out the SD card after every route, unless it's a short ride to the supermarket or something.
I hoped this helped OP or any other one looking for a dashcam solution.