What you are asking for is a workstation motherboard, which does exist. Filter by the sTR5 socket for AMD Threadripper chips, for example.

Unfortunately, as far as finding something cheaper than that, you're looking for a product that appeals to a very small to non-existent market demographic.

Most of the buyers who want workstation boards (companies) do not want a computer that requires assembly.

The demographic that builds their own PC is almost exclusively doing so to play games.

Everyone else who wants to use a computer wants a portable laptop.

The good news is that all the complaints you have about gaming boards are mostly cosmetic. There's nothing unreliable about gaming boards. They all support the specs they claim to support. You don't have to use any overclocking features (I don't). They are off by default.

If you want low idle power consumption, what you actually want is a system that has soldered RAM (LPDDR) which essentially goes against the other parameters of what you asked for. You don't want a module desktop PC at all if that's your parameter. What you really want is a mini PC or a Mac mini.

You're asking for a workstation board with low idle power consumption, but nobody who wants that is optimizing for low idle power consumption.

The best system for you is probably an HP, Dell, or Lenovo workstation PC. The good news for you is that these are all over eBay as corporations tend to sell them in bulk when they're done with them in just a few short years. They're reliable, quiet, and have low idle power consumption. Or, you can go with the big workstations that support ECC RAM.