The 4K H265 videos I've downloaded from SLR have all played fine on my Oculus Go, using either DeoVR or Gallery (sometimes it seems like gallery is smoother frame-rate wise, but I'm not sure). I always just pick the option that is listed for "Oculus Go / Quest" and it works.
That said, I think it was only recently that they changed from H264 to H265 on the Oculus Go, after someone did some tests and found the 5K H264 versions had more artifacts and looked worse than 4K H265, IIRC. I had assumed until SLR made the change that the Oculus Go wouldn't be able to handle the H265, but as I said, they've been fine for me. The reduction in file size is a nice bonus, too.
When you say "none of them play properly", what exactly happens? Is it jerky, or are there other problems? As I said, they work fine on mine, so it's hard to offer more advice. My Oculus Go is pretty new, though I doubt that should make a difference.
Do the 5K H264 files work fine on your Oculus Go? If so, sounds like you should stick with those for now - the quality difference between them and 4K H265 is not large, in my experience.