Firmware and SD is up to date.
This GoPro is a replacement, and honestly I TJ ink I should have kept the one I sent back as it had much less issues than the one I have now.
"flickering on the screen" what country are you in and is the camera set to 60Hz (NTSC) or 50Hz (PAL)?
"very visible and distorted block artefact" Did you record in HEVC? What are you using to play back/edit your video? What resolution/frame rate and digital lens are you seeing this in?
"sometimes the GP doesn’t turn on after charging" The first time it's turned on it can take longer. If it wont turn on at all, that doesn't sound right and it might be best to do another exchange.
"SD is up to date" Not sure what this means, I assume you mean it's on the recommended list. Try a different card from this list and see if you get the same results https://community.gopro.com/t5/en/SD-Cards-that-Work-with-GoPro-Cameras/ta-p/394308
I’m pretty sure it recorded in h.264 because I’m only recording 1080p. And about the and card, I’m using the as card that was bundled with the GoPro, so it’s their own recommendation.
"Is is also 100." I don't know what this means.
There is the option for HEVC and h.264 + HEVC. Which is it?
I'm not sure what you're looking for here. Are you looking for help, because direct answers will make that easier. Instead of saying, "right region frequency" just give the frequency and region, like asked. Same with things like the giving the actual name/model/size of the SD card.
Upload a video that has the "distorted block artefact" directly to youtube without editing and then post the link here so we can see what you are talking about. You can also post a video with the "weird flickering on the screen".
here is a start up. match it if you like see if it replicates yours