Totally separate supplies creates an issue with switching. Can the project accept one rail going high before the others by some relatively long period of time. It's pretty clear the FPGAs don't tolerate this kind of thing because they invented power management chips for this very reason.
I would want the isolation that is provided by a transformer design. In my view, transformerless is out of the question.
When you pick a preregulation DC voltage, consider the implications on the lower voltage regulators. The preregulated voltage for a 19V supply is probably close to 24V, maybe as little as 22V. Then think about the poor 3.3V regulator dropping 19V at 1A => 19 Watts. Can the regulator handle it? This is a bigger problem with 30V supplies but still worth thinking about.
That's why commercial supplies have transformers with multiple output taps.
For Op Amps, I would definitely prefer +-15V over +-12V because the older op amps just can't get within 3V of the rail and the signal swings are usually +-10V. Many of the older analog computers use +-10V for their signal swing and +-15V for the rails. The Comdyna GP-6 is an example:
https://oldcomputermuseum.com/comdyna_gp6.html