The name of the IC that runs a low-harmonic power supply

There are standardized methods for calculating reliability such as Bellcore Telcordia. Accuracy may be questioned, but they provide a basis for comparison.

I don't imagine there is any need for electrolytic capacitors in an SBC, as the power supply would likely best be separate, and there is little need for tantalum parts with the progress in MLCC caps.

Complex things like memory chips running very hot, and large numbers of MLCC bypass caps should probably figure into your lifetime calculations. Relays have a defined life, optoelectronics generally ages, but VLSI chips also have a temperature-dependent lifetime due to some mechanisms like metal migration. MLCCs can fail shorted.

And, if you're not selling a packaged bulletproof product, the lifetime may be foreshortened by user activities. I once received a "warranty repair" temperature controller that had failed due to a forklift tine penetrating the front panel to a depth of at least 6". They'll do stuff like connect 240VAC to ADC inputs and put electronics in lightning-prone locations.

A step down or buck regulator tends to be just as or better than a comparable step up or boost regulator, in terms of efficiency. Both refer to switching regulators. There are also buck-boost regulators that can do both, allowing it to regulate through the input voltage range of a battery supply.

You are confusing a switching step down regulator with a linear regulator, essentially a large resistor, who's efficiency goes down sharply as input - output voltage increases as well as current increases.