I have been using an M3-ATX PicoPSU for many years. I have used it both with a power brick on grid and connected to solar without issue. As far as running a motherboard with drives, CPU, and even a very small GPU it has worked well for me.
I would drop the CPU voltage on whatever CPU you choose. If you find a T model Intel for the same price as a regular, then go for it. Otherwise it isn’t worth it to spend extra when you can generally drop the voltage significantly. I have no idea how it works with AMD so YMMV. My entire system with a 53 watt CPU G1850 Celeron), HDD, SSD, a couple USB devices, and single fan for the CPU was using ~30 watts idle and ~40 watts max with the CPU voltage down to 0.9V which was as low as my cheap board would go.
I would recommend going 24V and using step-down power converters for a more powerful setup than what I use. If I remember correctly, the PicoPSU’s step voltage down to 12V when it is too high, but when it is within a certain range it just let’s it through unfiltered. Also, it would likely work out better to have the motherboard PSU set to power down the system when voltage gets too low (say 20 volts) but still plenty of voltage to keep things running until it has shut down properly. That way you could just use dumb step-down converters to power the GPU’s.
Another benefit to using 24V is the solar power setup. It is usually easier to find larger panels running at 24V. Also, most of the solar charge controllers are limited by how many amps they can handle. You can hook up twice the amount of watts at 24V than at 12V, although you would want to confirm this with the manual for any specific solar hardware you use.
Personally I want to move to 24V to make it a bit easier once USB-PD becomes more common. I intend on running lines for 5V, 12V, and 19V with step-down converters. Running monitors, charging laptops, and running any standard 12V or USB/5V devices will be simple and I won’t need any other voltages off-grid except for large appliances or tools which would require a dedicated power source with my setup.
This method will provide consistent and clean power until the charge controller or PC PSU shuts things down due to low voltage, instead of possibly burning things up that are fed too low of a voltage before power is cut. That’s just my take on things anyway. You can find industrial step-down converters capable of delivering the power you need to run all of the GPU’s you want. This would be substantially more efficient than running power inverters just to convert the power back to what you need and the hardware would likely cost less in the end as well.