Questions on making a minimal/custom Arduino circuit @ 3.3v/8Mhz?

what IS the minimal (but still somewhat stable/safe) components needed?

A minimum implementation would include just the chip itself.

A reasonable minimum implementation would have a decoupling cap, a crystal + two caps, or a decoupling network on the analog supply. And then more from there, based on your needs.

I have done many "chip-only" minimum implementations myself.