In some ways I would be happy if it was a motherboard issue, I'd finally do my upgrade and move on. It is a real pity that there isn't any kind of event log etc. within the Asus UEFI BIOS, that would be really helpful. I think I've seen that this is sometimes a feature on "corporate" workstation boards but it isn't on mine anyway.I'm not saying that the SF 750 is a bad psu, just that the motherboard has issues with voltages. The SF will probably work fine for years in a different system.
I bought the Maximus Gene VII two years ago from eBay and paid more than I should for it, I wanted it to see me through until 2023 and give me one of the best platforms for my 4790K during that time. But it has had these random power issues ever since I got it, and I'm getting a bit miffed with it now.
A lot hinges around the TJ08-E TBH - I really like the case and would like to keep it, but the PSU limitation (given that I also want an ODD) is a bummer. And it is pretty cramped in general, although once it is together and working (well, mostly) then that doesn't matter anyway. I really wish that Silverstone would release a "long wheel-base" version of the TJ08-E that is just an inch or two longer with some updated connectivity, but it will never happen.
EDIT - having said that, if the SF750 is actually fine then I could keep the case but I'd need another micro-ATX board and they don't seem quite as fully-featured these days from what I can see (my Gene VII has got 8 SATA ports).