From the kernel log, it looks like a v1. As I don't have a v1 and there is was a warning in the openwrt wiki about flashing a v1 without a serial console, (at least initially when coming from something other than openwrt), I'm hesitant to advise you here. I've seen others post about the v1 so maybe search the forums and see if an existing user is willing to advise you here.
EDIT: it seems the warning about using a serial console has been removed (others have speculated that this warning may be overblown - use your own judgment)
If it is a v1, I don't think you are going to get 5 GHz using openwrt regardless of what you do.
Only if you can afford to replace it and live without it:
I might try flashing netgear_r7500-squashfs-factory.img (a v1) image via tftp and see if that works and if the dmesg is any different. You can also try flashing a v1 "sysupgrade" image via luci web interface but that might be more risky.
After that I might then try flashing netgear_r7500v2-squashfs-factory.img via tftp.
If your unit is actually a v2 and it turns out you accidentally flashed a v1 image from the start, I'm pretty sure you have nothing to worry about. My experience (and others with r7800's - similar to the r7500v2) is that it is really hard to break it by tftp (or luci web interface) flashing the wrong thing.
If it turns out to be a v1, its still a great switch and a decent router. Only having 2.4 GHz would be a disappointment.
Lastly, I don't use mesh so you might search the forums and then open another post about that.