This is obnoxiously difficult to shop for in the desktop/workstation space.
Like, what are you actually connecting your desktop to?
The only reason laptops depend on Thunderbolt is because they have limited internal expansion and need high performance external I/O.
If you need more things than gaming boards offer then obviously you have very advanced needs and can go pay for a workstation board, something like an sTR5 socket Threadripper board.
I don't mind having to work within those physical limits but I do want to be able to search for boards that support N components. i.e 1x 4.0x8, 2x 3.0x8, 4x 5.0x4 . But the best you can search for is physical sizes of pcie slots and then dive into a spec sheet for each one, only to find that the 6 x16 slots only have 1.0x1 of bandwidth each.
Most people only need the PCI lanes for graphics cards and storage. There aren’t many other internally installed devices out there that actually need that kind of bandwidth, and a lot of those use cases are already covered by alternatives like Ethernet or USB, or they’re already on your board (m.2 slots, fast Ethernet ports).
The 6x16 slots with 1.0x1 bandwidth are there so that people can plug in stuff like sound cards and other random stuff that generally has pretty light bandwidth needs.
If I just search for “PCIe card” on Newegg most of the resulting products max out at x4, and most of the ones that do are already on the board (m.2 cards, additional USB/Thunderbolt).
The one use case that seemed useful and unusual in my search results was a quad port HD video capture card which seemed to require x4 bandwidth.
If you had a scenario like you describe where there isn’t a single x16 slot, you’ve instantly annoyed 95% of the market that needs that full bandwidth for a GPU, whether it be for gaming or for professional applications.
Some solutions that avoid expensive workstation boards and CPUs include getting a higher end chipset to get gaming boards that come with 2x x16 slots, or you can use accessories and adapters that just plug into m.2 slots.
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.